Washington (CNN) -- The fire that crippled the Carnival cruise ship Triumph started with a leak in a fuel-oil return line running from one of the ship's engines, 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 in a flexible hose section was intentional.
"Fire suppression was immediately activated by the crew, first by waterfog and then by (carbon dioxide). They did a very good job," Hatfield said.
"We are looking at the cause of the fire and why the ship was disabled for so long, and we are also looking at the crew response to the fire as well."
Hatfield said the investigation will last for several months. The Coast Guard said it has conducted 21 interviews with passengers and crew members since 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.
Vance Gulliksen, a spokesman with Carnival Cruise Lines, told CNN Monday that the ship's last scheduled Coast Guard inspection was on November 15. Gulliksen also said the cruise line agreed with the Coast Guard's determination of the origin of the fire.
Coast Guard spokesman Carlos Diaz said the line ran from the ship's "number-6 engine" to a fuel tank.
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.
Stranded on the crippled ship, passengers and crew lived with worsening conditions, as toilets stopped working, and waste spilled onto floors and into hallways. Passengers had to use plastic bags to collect their waste.
Passenger Cassie Terry described the ship as "a floating toilet, a floating Petri dish, a floating hell" in a lawsuit filed Friday against Carnival for unspecified damages related to the cruise.
Passengers reported long lines for food, shortages of fresh water and widespread boredom. Many passengers slept in hallways or outside to escape the odors and heat below decks.
Patrick Cuty, a senior marine investigator for the Coast Guard, told CNN Sunday that investigators had located the area where flames erupted in the engine room.
"We know that the fire originated in front of a generator," Cuty said. "You can see the ignition marks on the wall."
There are three generators in the engine room where the fire broke out. Three other generators are in a second engine room that wasn't involved in the fire, Cuty said.
The same ship encountered a problem in January with its propulsion system, according to a notice posted on the website of Carnival senior cruise director John Heald.
On Saturday, Carnival crew members were bused to and from the ship to help with the clean-up. One housekeeper told CNN it wasn't pleasant work but said it had to be done, and the crew was willing to do it.
Passengers have praised the crew for its response during the ordeal.
Because the Carnival Triumph is a Bahamian-flagged vessel, the Bahamas Maritime Authority is the primary investigative agency and will work with the Coast Guard and the National Transportation Safety Board.
Investigators pulled the voyage data recorder, a device that records alarms, voice communications on the bridge, engine speed, navigation information and rudder angle, Cuty said.
It appears that the fire suppression worked as designed, Cuty said Friday. The engineer who was on watch around dawn February 10 saw the fire ignite over a video feed and immediately notified the bridge, Cuty said.
CNN's Jim Barnett contributed to this report.