Traffic along Interstate 35 near Parmer Lane had slowed to a crawl late Thursday so cars could merge into one lane when a big rig barreled into them, causing a chain reaction that killed five people, federal investigators said Saturday.

The 18-wheeler, operated by a Dallas-area company, “failed to slow down” as it reached where vehicles were consolidating from three lanes into one because of a resurfacing project by the Texas Department of Transportation, according to National Transportation Safety Board officials.

Those killed, including an infant and a child, were inside two cars, NTSB investigators said.

“This is a difficult situation, and my heart and prayers go out to everyone involved,” NTSB Vice Chairman Alvin Brown told reporters Saturday afternoon in Austin.

Brown and investigator-in-charge Kenny Bragg said it is unclear at what speed the truck, driven by Solomun Weldekeal Araya, 37, was traveling when he hit the cars. Analysts will seek to determine how fast he was going using video from the scene and information stored on the truck.

The brief news conference offered the clearest insight into the cause of the crash, which also left the major thoroughfare closed nearly most of the day Friday. As many as 19 cars were hit during the crash, Austin police said in a search warrant affidavit.

NTSB officials did not discuss the criminal charges against Araya, who Austin police arrested on five charges of intoxication manslaughter and two counts of intoxication assault. He remains in the Travis County Jail.

The affidavit said that police think he was under the influence of depressants and showed signs of intoxication, including swaying and watery and bloodshot eyes. Araya tested 0.00 on a breath test, the affidavit said.

On Saturday, Amazon spokeswoman Maureen Lynch Vogel confirmed the driver was hauling a load for the company.

“This is a horrible tragedy, and our thoughts are with all those involved. We’re looking into this further and will cooperate with all investigations,” she said in a statement.

A person who answered the phone at ZBN Transport confirmed that Araya, who lives in Dallas, was his employee and had worked for the company for about four months.

NTSB officials said their investigation will consider broader issues that may have contributed to the crash and will likely issue a preliminary report in 30 days. A final report could take up to two years.

Bragg said the agency’s effort is to “not just what happened and how it happened and to recommend changes to prevent it from happening again.”

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A man was charged Friday with intoxication manslaughter after five people were killed and several injured in a late-night wreck in Austin, Texas, that involved over a dozen vehicles on Interstate 35, authorities said.

Authorities said that the five people killed in the crash that involved 17 vehicles Thursday just before 11:30 p.m. included three adults, a child and an infant. First responders said that 11 people were taken to hospitals.

Solomun Weldekeal Araya, 37, was charged with five counts of intoxication manslaughter and two counts of intoxication assault after the crash, Austin police said Friday. Police said he was in custody in Travis County Jail. It was not immediately clear if he had an attorney. Jail records did not list an attorney for him.

Police have not detailed the circumstances that led to the wreck. Police said in an email Friday that they were still early in the investigation and had no further information available to release.

The southbound lanes of I-35 were closed following the crash, and they remained closed into Friday before reopening at about 1 p.m. The wreck left a stretch of the interstate littered with mangled vehicles and debris.

The collision was “very large and very complex,” police Officer Austin Zarling said at an early morning news conference.

Edgar Viera told NBC affiliate KXAN of Austin that he was at a nearby store when he heard the crash and went to try to help those involved.

“We didn’t have the proper tools to open the vehicles, so we just did what we could,” Viera told the station. “It was hard to see this.”