The AP has the story here:
The takeaways:
A Coast Guard official said Monday that a leak in a fuel oil return line caused the engine-room fire that disabled the Carnival Triumph in the Gulf of Mexico, leaving 4,200 people without power or working toilets for five days.
Cmdr. Teresa Hatfield addressed the finding in a conference call with reporters. She estimated that the investigation of the disabled ship would take six months.
and:
She said the crew responded appropriately to the fire. “They did a very good job,” she said.
In an email after Monday’s conference call, Coast Guard spokesman Carlos Diaz described the oil return line that leaked as stretching from the ship’s No. 6 engine to the fuel tank.
…
Cruise industry expert Andrew Coggins, a former Navy commander who was a chief engineer and is now a professor at Pace University in New York, said the fire could potentially have been serious.
“The problem is the oil’s under pressure,” he said. “What happens in the case of a fuel oil leak where you have a fire like that is it leaks in such a way that it sprays out in a mist. In the engine room you have many hot surfaces, so once the mist hits a hot surface it will flash into flame.”
If the crew hadn’t reacted quickly and the fire suppression system hadn’t worked properly, he said, “the fire from the engine room would have eventually burned through to other parts of the ship.” Engine room fires that can’t be suppressed generally result in the loss of the entire ship, he said.