SAN FRANCISCO — Wrapping up more than three years of work that cost the U.S. government $2.3 billion, Bechtel Corp. is leaving Iraq with a subdued sense of accomplishment after suffering through a spree of violence that killed 52 workers.

The departure of the San Francisco-based engineering firm serves as another sobering reminder of how the carnage in Iraq has scrambled the United States' once-grand ambitions to rebuild the devastated country.

The U.S. government hired Bechtel in April 2003, hoping the company behind marvels like the Hoover Dam would be able to bring Iraq into the 21st century as it repaired damage caused by the invasion that overthrew Saddam Hussein.

The daunting task required rebuilding roads and bridges, expanding the power grid, cleaning up the water supply and adding telephone lines.

Although progress has been made, the efforts of Bechtel and other government contractors have been beset by sabotage, corruption and lawlessness that made it difficult to retain workers frightened for their lives.

Bechtel said it completed all but two of the 99 projects on its Iraq to-do list, but at a cost of 52 workers killed and 49 wounded. At peak times, Bechtel employed more than 40,000 workers — mostly Iraqi subcontractors — on the various projects.

Most of the Bechtel workers were killed while off duty, said company spokesman Jonathan Marshall. It's among the greatest losses of life that Bechtel has suffered during any job in the company's 108-year history, possibly exceeded only by the company's Depression-era work on the Hoover Dam, Marshall said.

Bechtel has completed more than 22,000 projects in 140 countries, including the Channel Tunnel between Britain and France.

The Iraq contracts are a relatively small portion of Bechtel's revenue, which totaled $51.8 billion from 2003 through 2005. The privately held company does not disclose profits.