At least five people were killed Monday when a small Mexican Navy plane carrying a young medical patient and seven others crashed near Galveston, 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 of 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 80.5 kilometers (50 miles) 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.