The commandant of the Coast Guard says it's difficult to understand how a Marine helicopter and a Coast Guard airplane collided off the San Diego coast, killing nine people.

Adm. Thad Allen spoke Friday at Coast Guard Air Station Sacramento during a memorial service for the seven Coast Guard members killed when their C-130 plane collided Oct. 29 on a mission to rescue a boater. Two Marines who were training on a AH-1W Super Cobra helicopter also died.

The Coast Guard members were based in Sacramento, where they were eulogized as smart, caring people who loved to fly and volunteered time in the community.

Investigators are looking into the cause of the collision.