MIAMI (AP) -- Federal authorities tried to determine the age of a boiler on Monday as they investigated what caused a cruise ship explosion that left four crew members dead and injured more than a dozen others.
Investigators from the National Transportation Safety Board would not speculate on the cause of the explosion Sunday on the Norway, a 42-year-old Norwegian Cruise Line vessel. It remained docked in the Port of Miami.
The injured crew members remained hospitalized Monday, two in critical condition. None of the 2,135 passengers was hurt and terrorism is not suspected.
The victims were identified by Miami-Dade County police as Ricardo Rosal, 50; Rene Villanueva, 28; Ramil Bernal, 26; and Candido Valenzuela, 49. All were from the Philippines.
NTSB board member Carol Carmody said a key factor in the explosion is the boiler's age. Coast Guard and police officials have said it appeared that an accidental steam leak sparked the explosion.
The boiler that exploded had heavy maintenance in 1999 and underwent a routine cleaning and checkup on May 15 after about 3,000 hours of use, Carmody said.
The same day as the cleaning, the Norway passed its routine Coast Guard inspection, which checks for problems with a ship's engines and safety systems, officials said. The inspection does not cover boiler rooms on ships registered outside of the United States, such as the Norway, said Coast Guard Petty Officer Gene Smith. The Norway is registered in Panama.
Norwegian Cruise Line spokeswoman Susan Robison said she could not immediately determine how old the boiler was or how comprehensive the last checkup was.
Carmody said the chief engineer told investigators there was no fire on the ship, despite early reports. On two decks, the sprinkler system was set off by vibrations from the explosion.
NTSB inspectors will likely remain in Miami for up to two weeks, but a definitive cause of the explosion could take up to a year to determine, Carmody said.
It was not the first problem for the ship, which has been churning the oceans for the better part of four decades. During Memorial Day weekend two years ago, Coast Guard inspectors discovered about 100 ``soft patches'' covering holes in fire sprinkler lines that authorities said could have failed under intense heat.
Formerly the SS France, the ship was the longest _ if not the largest _ passenger liner afloat in the early 1960s. In 1974, the France was deemed unprofitable and taken out of service. It was eventually bought by Norwegian in 1979 for $18 million _ its value in scrap metal _ and revamped at a cost of $120 million.