MEXICO (REUTERS) - The fire on board the Carnival Corp cruise ship that drifted for days in the Gulf of Mexico awash in raw sewage started from fuel from a leaking diesel engine line that ignited, the United States (US) Coast Guard said on Monday.

More than 4,200 passenger and crew were stranded for five days last week on board the Triumph cruise ship, a journey followed closely by US cable television news stations that regaled audiences with details about backed-up plumbing and overflowing toilets on board the damaged vessel.

The ship was towed into port in Mobile, Alabama, late on Thursday.

The fuel leaked from a diesel fuel oil return line that was touching a hot surface and caught fire, said US Coast Guard Lieutenant Commander Teresa Hatfield in a conference call with reporters.

The investigation into the incident, which also is being studied by the National Transportation Safety Board, is likely to take as long as six months, she said.

There was no indication the fire was intentional, she said.