A small Mexican Navy plane transporting a young medical patient and seven others crashed Monday near Galveston, killing at least five people and setting off a search in the waters off the Texas coast, officials said.

Four of the people aboard were Navy officers and four were civilians, including a child, Mexico’s Navy said in a statement to The Associated Press. It was not immediately clear which of them were confirmed dead by U.S. Coast Guard Petty Officer Luke Baker.

Two of the people aboard were members from the Michou and Mau Foundation, a nonprofit that provides aid to Mexican children with severe burns.

The crash took place Monday afternoon near the base of a causeway, a raised roadway typically built over water, near Galveston, along the Texas coast about 50 miles (80.5 kilometers) southeast of Houston. The cause is under investigation.

Mexico’s Navy said in a statement that the plane was helping with a medical mission and had an “accident.” It promised to investigate the cause and is helping local authorities with the search and rescue operation.

Teams from the Federal Aviation Administration and National Transportation Safety Board have arrived at the scene of the crash, the Texas Department of Public Safety said on the social platform X.

A spokesperson from NTSB said they are “aware of this accident and are gathering information about it.”

The Galveston County Sheriff’s Office said officials from its dive team, crime scene unit, drone unit and patrol were responding to the crash. It said in a post on Facebook that the public should avoid the area so emergency responders can work safely.

Galveston is an island that is a popular beach destination.

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.