By Mike M. Ahlers and Sandra Endo
WASHINGTON (CNN) -- The fire that crippled the Carnival cruise ship Triumph started with a leak in a fuel-oil return line connected to the ship's No. 6 generator, the U.S. Coast Guard said Monday.
Leaking oil hit a hot surface, starting the fire, said Teresa Hatfield, the lead investigator for the Coast Guard. Hatfield said there was no indication the leak was intentional.
Hatfield said the investigation will last for several months. The Coast Guard said it has conducted 21 interviews with passengers and crew members since last Thursday, when investigators boarded the ship while it was still at sea.
Hatfield said the oil return line is one of the items that is routinely inspected, but she did not say when it was last inspected or describe its condition at that time. She praised the crew for doing a "very good job" containing the fire.
The Triumph was on the third day of a planned four-day cruise from Galveston, Texas, to Mexico when the fire broke out and brought the trip to a halt. It was carrying more than 4,200 people, including 3,100 passengers. The Triumph was eventually towed into port in Mobile, Alabama, Thursday night, and the last passengers disembarked Friday.
-- CNN's Jim Barnett contributed to this report.