Six sailors and a civilian firefighter injured in an explosion during boat maintenance at a New Jersey naval base have been released from hospitals. One sailor remains hospitalized after surgery for a broken arm.

U.S. Navy officials said they are still trying to determine what caused Tuesday morning's blast at the Naval Weapons Station Earle in Middletown Twp., Monmouth Co.

The explosion occurred while personnel were doing routine maintenance on a 27-foot aluminum utility boat, which is used to support port operations at the base, said Navy spokeswoman Beth Baker.

The blast was confined to a boathouse where the maintenance was being done.

Naval Weapons Station Earle provides ammunition and explosives for vessels. No ammunition was involved in the explosion, the Navy said.