The investigation will now focus on the cause, the crew's response and why the ship was disabled for so long.
The fire disabled the Carnival Triumph in the Gulf of Mexico, leaving 4,200 people without power or working toilets for five days.
The Triumph left Galveston, Texas, on Feb. 7 for a four-day trip to Mexico. The fire paralyzed the ship early Feb. 10, leaving it adrift in the Gulf of Mexico until tugboats towed it to Mobile, Ala., on Thursday.
Passengers described nightmarish conditions on board: overflowing toilets, long lines for a short supply of food, foul odors, and tent cities where vacationers slept on deck.
The Associated Press contributed to this report.