The firefighter scooped up the figure slumped on the kitchen floor and dashed for the ambulance waiting on Blake Street. As he moved through the smoky haze, he was struck by a thought that is still with him: It was like nothing was in his arms.

As the ambulance sped toward the hospital, emergency medical technicians administered oxygen; one reflexively commented on the overpowering smell. Right away, as if to apologize, the patient spoke up. It had been more than a year since he had been permitted to shower, he said.

A police officer in the vehicle leaned in.

The patient started speaking and did not stop. He gave his name, said that he was 32 years old and had spent most of his life held captive by his father and stepmother, who locked him in his room for some 23 hours a day.

At the hospital, he continued his story. He had been trapped for two decades, forced to defecate into newspapers and to funnel his urine out the second-story window. He hadn’t seen a doctor or a dentist in 20 years. Sometimes he was fed a sandwich. His teeth were so decayed they often broke when he ate. He was 5-foot-9, but he weighed only 68 pounds.

The ride in the ambulance, he said, was the first time he had been let out of the house since he was 12.

Then, he made a confession. He was the one who set the fire. He used a lighter forgotten in the pocket of an old jacket that his stepmother had given him. If he did not die in the fire, he had reasoned, he might finally be set free.

The revelations that began in that ambulance ride on Feb. 17 cracked wide one of the most shocking secrets to ever tarnish Waterbury, a small, former manufacturing city in the southern part of Connecticut. The police now believe what the man said in the ambulance that evening: For the past 20 years, an 8- by 9-foot room on the top floor of a disheveled house at 2 Blake Street was a prison cell for a boy — now a man — last been seen by the outside world when he was in the fourth grade.

But many in the community had feared for the boy’s safety for a long time.

For years before the man’s disappearance, his teachers, classmates, neighbors and his elementary school principal all believed he was suffering silently. They repeatedly called the Waterbury Police and the Connecticut Department of Children and Families to intercede for a child they said was so hungry that he ate from the trash and stole his classmates’ food.

Many reports that may have documented these calls have since been lost, but what records remain show that responding authorities determined the boy was doing OK.

After a while, without turning up any evidence of abuse, the calls stopped coming. In fact, until the fire, the last recorded police visit concerning the boy on Blake Street was April 18, 2005, in response to a call placed by his own father. He summoned officers to complain that he was being harassed by people continually checking up on his child.

That year, his son was pulled from school, purportedly to be educated at home. In interviews with police officers last month, the man told them that for a brief time he received school work sheets, but all formal education stopped shortly after. The next time he left his home, 20 years later, it was in the arms of the firefighter.

“He looked,” said Detective Steve Brownell of the Waterbury Police Department, who interviewed him later at the hospital, “like a Holocaust survivor.”

Late last month, the man’s stepmother, Kimberly Sullivan, 57, was arraigned in Waterbury Superior Court. She has been charged with kidnapping, assault, cruelty, unlawful restraint and reckless endangerment. If convicted of all charges, she could serve the rest of her life in prison. Last month, she pleaded not guilty.

“She is adamant that she had done nothing wrong,” her lawyer, Ioannis Kaloidis, said in an interview. Mr. Kaloidis laid blame on the biological father, Kregg Sullivan, who died in January of last year. (The biological mother had given up her parental rights to Mr. Sullivan, to whom she was briefly married.)

“They make it look as if Kim Sullivan made all the decisions, that she pulled him out of school, that she decided what he was or wasn’t going to eat, that she decided when he went to the doctor,” Mr. Kaloidis added. “She was not the child’s mother.”

Speaking at a news conference last week, Mr. Kaloidis disputed the man’s claims of captivity. “Where are the handcuffs,” he asked. “Where are the chains? Where are the signs of restraint? It doesn’t add up.”

In addition to her stepson, Mrs. Sullivan also had two younger daughters with Mr. Sullivan — Alissa, now 29, and Jamie, now 27 — who seemed to be free to come and go as they pleased. (The women’s biological relationships to the Sullivans are unclear.) In fact, several neighbors on Blake Street said they never knew there was a third child.

The man, who is recuperating at a Connecticut medical center, has not yet made a public statement. (The police have not released a photograph of him, nor have they released his name because they say he is a victim of domestic abuse.) A conservator, whose identity has not been made public, has been appointed by the court to protect the man’s interests. The Times asked both the Waterbury mayor and the state’s attorney to forward requests for comment to the man but received no response.

At a hearing where a judge ordered Mrs. Sullivan to wear an ankle monitor while on bail, Donald E. Therkildsen Jr., an attorney representing the state, told the court that when he met with the victim, “his first question was, ‘Why is she out walking around while I was locked up in a room for 20 years?’”

As he heals, the city is grappling with the failure of the authorities who were entrusted to help him. And another, more troubling question persists: How could this have happened to a child that so many people were so worried about?

‘I Have Been Kept a Secret My Entire Life’

Tom Pannone, who was a principal at Barnard Elementary School, says he can still remember the uneasy feeling he had about the child who was enrolled at his school in 2001. The boy arrived daily with a dirty plastic lunchbox, he said; at least once, Mr. Pannone found him in a bathroom before school started, devouring his packed lunch. It was there that he saw the boy standing at a urinal, drinking the water as he flushed. Mr. Pannone called the boy’s stepmother, he said, and the behavior stopped.

But the child was still always hungry and disheveled. Over the five years the boy attended Barnard Elementary, Mr. Pannone said he made call after call to the Department of Children and Families. Each time, he said, they would investigate and report back that the child was fine.

“You knew something was not right,” Mr. Pannone said in a recent interview. “He appeared to be a happy-go-lucky kid, but we knew that something was amiss.”

Even after the boy was pulled from Barnard Elementary to be home-schooled, Mr. Pannone said, he was worried. So he routinely sent attendance counselors to the house on Blake Street — technically, he reasoned, since the boy was not enrolled elsewhere, he was still a Barnard student. He also informed the police of his concerns, he said.

Police records indicate at least two calls to the house after the boy had been withdrawn from school. One, on April 1, 2005, was placed, according to the police report, by his classmates, who were afraid “that he may have died, because he has been out of school for so long.” Mrs. Sullivan told responding officers that her son was being home-schooled.

Frustrated, Principal Pannone tried another way.

He asked for the help of the Lopes family, who lived right next door to the Sullivans and whose son, Peter, was then a 10-year-old Barnard student. Mr. Pannone asked Peter and his family to keep an eye on their neighbor.

Peter Lopes, who is now 29, has not lived in the neighborhood since 2009, but said he remembered the last time he saw his former classmate. It was shortly after the boy was pulled out of Barnard. He can still picture a too-skinny kid with an infectious smile. The boy was standing on the peeling porch next door, but ventured no further.

“I said, ‘Where have you been?’” Mr. Lopes recalled. “I’m home-schooled,” was his answer.

In a warrant for Mrs. Sullivan’s arrest, the man said that his stepmother and his father forbade him to have friends. “I have been kept a secret my entire life,” he told the police.

Principal Pannone was not the only one trying to uncover that secret: For decades, the man’s half sister, Heather Tessman, whom their biological mother had given up for adoption before her son was born, fruitlessly dug through yearbooks of local schools she found online, she told The Times, hunting for the brother she had met once, when she was 3 years old.

“You can’t find a person who doesn’t exist,” Ms. Tessman, 35, who lives in Vernon, Conn., said in an interview.

“He didn’t get to see a movie. He didn’t get to go to a concert, he didn’t get to fall in love and get his heart broken,” she added. “It kills me.”

Counting Cars

Inside his room, which was secured with a slide lock from the outside, the man read and reread a handful of books, he told the police, looking up words he didn’t know in a dictionary. He “ultimately educated himself,” the police affidavit reads.

He escaped once. In 2005, when he was 12 or 13, he broke off a piece of the door’s center paneling; but rather than fleeing the house, he simply slipped down to the kitchen to scrounge for food. When his breakout was discovered, he told the police, his bedroom door was reinforced with plywood. Threats of withholding food, or violence, kept him from trying again.

The door, with its locks and plywood reinforcement, has been entered into evidence in the case. It was “clearly meant to keep someone in, not someone out of the room,” the police document said.

He kept track of the year by the snatches of radio he would overhear, following NASCAR races and University of Connecticut basketball. Mostly though, he looked out the window and counted the cars passing on Blake Street.

To the outside world, the Sullivans were a family of four — that is, Mr. and Mrs. Sullivan and their two daughters. Goodwin Lowe, 73, a clerical worker who has lived a few doors down since 2008, said that over the years, he would watch the girls playing in their yard and jumping on their trampoline from his patio.

“I never knew there was a boy in that house,” Mr. Lowe said.

It is unknown what conditions the daughters were raised in, or what knowledge they had of their stepbrother’s condition. The daughters have not been charged with any crime. Neither responded to calls for comment. None of Mr. or Mrs. Sullivan’s several siblings responded to text messages, calls or requests for interviews at their front doors.

Mr. Sullivan worked for 30 years in shipping at Gem Manufacturing, an industrial parts producer in Waterbury, according to a 2007 bankruptcy filing by his wife. Mrs. Sullivan was a retail clerk, working for a time at the perfume counter at a Macy’s at the local Westfarms Mall, according to a former supervisor there, Lee Wassell.

Several years ago, Mr. Wassell said, Mrs. Sullivan revealed that her husband had suffered a stroke and was using a wheelchair. She complained of being burdened with his care, Mr. Wassell said. She often talked about her daughters, he said, but never mentioned that she had a stepson.

For a time, the boy was allowed out of his room for maybe an hour a day to do chores. He stepped outdoors only to take a family dog to relieve itself in the back yard, he told police, excursions that lasted about a minute. Sometimes, when his stepmother was out of the house, his father let him out of his room to watch television together.

After his father’s death, the man told the police, his confinement to his room became near total.

A Lighter and Hand Sanitizer

The day of his escape was not planned. On one of his brief outings from his room, he had swiped a bottle of hand sanitizer and read on the label that it was flammable. With the lighter he had once scavenged from his late father’s jacket pocket, he set fire to a pile of printer paper, he told police. He waited until the blaze grew out of control before calling for help.

His stepmother unlocked his door and he fled downstairs, where he collapsed. According to a police report, two other people arrived at the house just then, and as the man lay there, he overheard his stepmother “yelling to them to get a screwdriver to get the locks off the door” before the fire department arrived. (The names of the two people have not been released.) His stepmother demanded he get up and wash his face, he told detectives; she didn’t want anyone to see how filthy he was.

He did not listen to her.

“He purposefully didn’t get up so the fire department would be forced to get him,” the affidavit read. He “believed this was the only way out of his situation.”

This was how Gabriel Goja, a firefighter with the Waterbury Fire Department, found him at 8:42 p.m. on Feb. 17. “For him to choose that way to get saved, it’s heroic,” Firefighter Goja, 35, said in an interview. “To get to safety by trusting us to save him — he saved himself.”

Since the fire, the man has been ensconced in a hospital rehabilitation center, according to Amanda Nardozzi, the executive director of Safe Haven of Greater Waterbury, a nonprofit organization that has been helping coordinate his care.

According to Ms. Nardozzi, he will need extensive physical rehabilitation — court documents state he has deformed knees and muscle wasting — and a carefully managed diet to avoid re-feeding syndrome, where a sudden flood of nutrients can kill a person near starvation. He is also receiving mental health counseling, Ms. Nardozzi said, funded in part by an official GoFundMe that has already raised over $200,000.

Three decades ago Tracy Vallerand, the man’s biological mother, gave up custody of her infant to the boy’s father. It was a hard time in her life, said Ms. Vallerand, now 52, and a diesel mechanic in Meriden, Conn. She said she had believed that her baby would have a better life with her ex-husband, Mr. Sullivan. When he remarried and moved, she said, he did not tell her where.

The next time she saw her child, she said, was on the body-worn-camera footage of his rescue from the house on Blake Street. Since then she has pored over reports of him and sat in court at Mrs. Sullivan’s hearings. But she has not been able to contact her son.

“I have cried and cried and cried and screamed and it makes me feel better for a little bit, but reality is that this has been a wide-awake nightmare,” Ms. Vallerand said in an interview.

“But I’m so proud of him,” she added. “I can’t say it enough.”

Susan C. Beachy contributed research.

Audio produced by Sarah Diamond.