A light aircraft operated by the Mexican Navy crashed near Galveston, Texas, on Monday while transporting a child for medical care, killing at least five people and prompting a search in nearby coastal waters, authorities said.

The plane was carrying eight people in total—four Mexican Navy officers and four civilians, including a child—according to Mexico’s Navy. U.S. officials did not immediately specify which passengers were among the confirmed fatalities.

Two of those on board were affiliated with the Michou and Mau Foundation, a nonprofit organization that helps Mexican children suffering from severe burns.

The crash occurred Monday afternoon near the base of a causeway close to Galveston, roughly 50 miles southeast of Houston. Officials said the cause of the incident has not yet been determined.

In a statement, Mexico’s Navy said the aircraft was conducting a medical mission when it was involved in an accident. The Navy said it would investigate the incident and is cooperating with U.S. authorities in search-and-rescue efforts.

Teams from the Federal Aviation Administration and the National Transportation Safety Board responded to the scene, according to the Texas Department of Public Safety. An NTSB spokesperson confirmed the agency is aware of the crash and is collecting information.

The Galveston County Sheriff’s Office said multiple units—including dive teams, patrol officers, drones, and crime scene investigators—were deployed. Authorities asked the public to stay clear of the area so emergency crews could operate safely.

Officials have not confirmed whether weather played a role, though the region has experienced fog in recent days. A National Weather Service meteorologist said visibility dropped to about half a mile around the time of the crash.