The Navy is investigating how a man was able to sneak on board an American aircraft carrier in port in England last month.

On April 9, the man made it past British and American security at Portsmouth, then apparently onto a liberty boat that was transporting people to the nuclear-powered aircraft carrier USS Harry S. Truman (search). The vessel was not connected by a walkway to land, Navy officials said.

The man was detained by members of the carrier's security force after he came on board, said Lt. Mike Kafka (search), a Navy spokesman in Norfolk, Va., where the Truman is based.

Another Navy official said the man was on board for 15 to 20 minutes.

After a search by security forces, officials concluded he hadn't done any damage or left any items on board, Kafka said. He was transferred to British custody.

"The military takes it very seriously when unauthorized individuals board our property or vessels," Kafka said.

British press reports identified the man as Abdoul Masmoud Yessoufou, 37, from the Isle of Wight (search). They said he was of west African origins and that he has been previously detained for entering restricted areas at Heathrow airport.

Navy officials said they had no indication that terrorism was involved.

U.S. Navy vessels have been on Al Qaeda's target list since 2000, when operatives rammed a boat loaded with explosives into the destroyer USS Cole while it was in port in Yemen, killing 17 sailors. The Navy then implemented measures aimed at increasing the physical security of ships in port.

The Truman was returning from operations in the Mideast when it stopped at Portsmouth. It has since arrived in Norfolk.