US authorities are today trying to determine the age of a ship boiler as they investigated what caused a cruise ship explosion that left four Filipino crew members dead and injured more than a dozen others.

Investigators from the US National Transportation Safety Board (NTSB) would not speculate on the cause of Sunday's explosion on Norway, a 42-year-old Norwegian Cruise Line vessel. It remains docked in the Port of Miami.

The victims were identified as Ricardo Rosal, 50, Rene Villanueva, 28, Ramil Bernal, 26, and Candido Valenzuela, 49. All were from the Philippines.

Formerly known as SS France, the liner operated a North Atlantic service, calling at Southampton from 1962 until 1974, when she was withdrawn amid rising costs and industrial disputes.

The vessel was brought for £45m in 1979 and converted to her present role as a Caribbean cruise ship.