Air traffic controllers lost communication for about 10 minutes with a small Mexican Navy plane carrying a young medical patient and seven others before it crashed off the Texas coast, killing at least five people, Mexico’s president said Tuesday.

Authorities initially believed the plane had landed safely at its destination in Galveston, near Houston, before learning it had gone down Monday afternoon, Mexican President Claudia Sheinbaum said. The cause of the crash remains under investigation. A search-and-resuce operation in waters near Galveston pulled two survivors from the plane’s wreckage, Mexico’s Navy said, while one remained missing.

Four of the eight people aboard were Navy officers and four were civilians, including a child, Mexico’s Navy said. Two of the passengers were affiliated with a nonprofit that helps transport Mexican children with severe burns to a hospital in Galveston.

“My condolences to the families of the sailors who unfortunately died in this accident and to the people who were traveling on board,” Sheinbaum said in her morning press briefing, without elaborating on a possible cause. “What happened is very tragic.”

U.S. Coast Guard Petty Officer Luke Baker said at least five aboard had died but did not identify which passengers.

The plane crashed Monday afternoon in a bay near the base of the causeway connecting Galveston Island to the mainland. Emergency responders rushed to the scene near the popular beach destination about 50 miles (80 kilometers) southeast of Houston.

Sky Decker, a professional yacht captain who lives about a mile (1.6 kilometers) from the crash site, said he jumped in his boat to see if he could help. He picked up two police officers who guided him through thick fog to a nearly submerged plane. Decker jumped into the water and found a badly injured woman trapped beneath chairs and other debris.

“I couldn’t believe. She had maybe 3 inches of air gap to breathe in,” he said. “And there was jet fuel in there mixed with the water, fumes real bad. She was really fighting for her life.”

He said he also pulled out a man seated in front of her who had already died. Both were wearing civilian clothes.

It’s not immediately clear if weather was a factor. The area has been experiencing foggy conditions over the past few days, according to Cameron Batiste, a National Weather Service meteorologist. He said that at about 2:30 p.m. Monday a fog came in that had about a half-mile visibility.

Mexico’s Navy said the plane was helping with a medical mission in coordination with the Michou and Mau Foundation. In a social media post, the foundation said: “We express our deepest solidarity with the families in light of these events. We share their grief with respect and compassion, honoring their memory and reaffirming our commitment to providing humane, sensitive, and dignified care to children with burns.”

Teams from the Federal Aviation Administration and National Transportation Safety Board were at the crash site Monday, the Texas Department of Public Safety said, and a spokesperson for the NTSB said the agency was gathering information about the crash. The Galveston County Sheriff’s Office said officials from its dive team, crime scene unit, drone unit and patrol responded the crash.