Luke Baker, a US Coast Guard petty officer, said at least five people had died, though officials did not say which passengers were among the victims. The cause of the crash is under investigation.
Mexico’s Marines said it was sending “its deepest condolences to the families of those who lost their lives in this tragic accident”.
Emergency responders and search teams were dispatched to the scene near the popular beach destination about 80km southeast of Houston. The aircraft was found largely submerged in the bay.
Sky Decker, a professional yacht captain who lives nearby, said he rushed out in his boat after hearing about the crash. He said he collected two police officers and was guided through thick fog to the wreckage.
Decker said he entered the water and discovered a badly injured woman trapped beneath chairs and other debris inside the aircraft.
“I couldn’t believe. She had maybe three 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.”
Decker said he also recovered the body of a man seated in front of her who had already died.
Medical mission
Mexico’s Navy said the flight was part of a medical mission conducted in co-ordination with the Michou and Mau Foundation, which provides emergency transport for children with life-threatening burns to Shriners Children’s Hospital in Galveston.
The foundation said in a social media post: “We express our deepest solidarity with the families in light of these events. We share their grief with respect and compassion, honouring their memory and reaffirming our commitment to providing humane, sensitive and dignified care to children with burns.”
A separate statement from Mexico’s Navy said the plane had an “accident” during its approach to Galveston, without giving further details.
A National Transportation Safety Board spokesman said investigators were “aware of this accident and are gathering information about it”.
It remains unclear if the weather played a role in the crash. The area has experienced foggy conditions in recent days, with visibility about 800m at the time.
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