SAN DIEGO, Dec. 21 (UPI) — An 8-year-old boy died and five adults were seriously injured when their pleasure boat collided with a U.S. Coast Guard vessel off San Diego, officials said.

The Coast Guard vessel, responding to a report of a grounded boat, crashed against the pleasure craft with 13 people on board Sunday, the Los Angeles Times reported.

The cause of the accident was under investigation, officials said.

The recreational boat was participating in the annual San Diego Bay parade of lights, when holiday-themed boats move in a procession around the bay, the Times reported Sunday. A member of the planning committee said he did not think the boat was a registered participant.

All six victims were taken to University of California-San Diego Medical Center, where the boy died of his wounds, a San Diego Fire-Rescue spokesman said. Another Coast Guard boat in the area took the remaining passengers to shore.

Both boats involved in the accident were taken to shore to be assessed for damage, Coast Guard spokeswoman Jetta Disco said. The size and type of the recreational boat were unknown.