LOS ANGELES, July 24 (Reuters) - A deadly 2007 coal mine collapse in Utah was triggered by a faulty mine design that left support pillars badly overburdened, federal investigators said on Thursday after a year-long investigation into the disaster.

Six miners were killed in the collapse of the Crandall Canyon Mine in a remote part of central Utah on Aug. 6, 2007. Three rescuers died during a subsequent collapse 10 days later.

In announcing its findings, the Mine Safety and Health Administration fined the mine operator Genwal Resources Inc $1.6 million for nine violations of its safety code. Genwal's engineering firm, Agapito Associates Inc., was fined $220,000.

The collapse at the Crandall Canyon mine sparked intense media coverage as rescuers below ground and from above worked to reach the six trapped miners.

The mine's owner said at the time that the collapse was due to an earthquake in the area but federal investigators said on Thursday that no such quake was responsible. The collapse itself was registered as a magnitude 3.9 seismic event.

"It was a mine design issue and the design created pillars that were simply not large enough to support the load," MSHA investigator Richard Gates told reporters at a news briefing in Utah to announce the findings. (Reporting by Dan Whitcomb, Editing by Sandra Maler)