MIAMI — A boiler explosion that killed eight crew members on what was once the world's largest passenger ship was likely caused by Norwegian Cruise Line's improper operation, maintenance and inspection of the steam chamber, federal investigators said.

Part of the boiler on the SS Norway had "extensive fatigue cracking" and deteriorated materials that weakened the metal and caused it to rupture under the intense steam pressure in May 2003, the National Transportation Safety Board said in a report quietly adopted in October.

The line's only steam-powered ship was docked in Miami when the boiler exploded and spewed steam, smoke and debris through engineering spaces, burning eight crew members seriously enough to kill them.

Seventeen workers were injured, but none of the 2,135 passengers was hurt.

Evidence "indicates that NCL shipboard engineers were aware of the cracking condition but did not take appropriate action to fix the problem," the report said.

The Miami-based cruise line, owned by Malaysia's Star Cruises and private equity firm Apollo Management LP, said it has improved safety procedures that are audited by the U.S. Coast Guard and took responsibility for the accident.

"None of the management or contractors who were involved in the operation, maintenance or inspection of the Norway's boilers are employed by NCL," line CEO and President Colin Veitch said in a statement.

The ship sustained about $20 million of damage from the blast and was retired from service. It is now being scrapped off the coast of India, despite opposition from environmentalists who claim it is full of toxic materials.

Also known as the Blue Lady, it was made in France and launched in 1960.