A crane being used for work on the new eastern span of the Bay Bridge toppled over after dropping a 129-ton piece of metal it was removing from underneath the bridge, a Metropolitan Transportation Commission spokesman said.

The barge-mounted crane was working in tandem with another crane to lower a section of the bridge's temporary support structure via a large basket onto the barge near the tower under the deck of the new self-anchored suspension span, MTC spokesman John Goodwin said.

For reasons that have yet to be determined, at about 12:15 p.m., the 258,000-pound temporary structure fell out of the basket and onto the empty barge below, causing the crane and its 168-foot boom to lose its center of gravity and also topple over, Goodwin said.

The temporary steel structure punctured the barge but did not sink it. There were four people conducting the work -- two crane operators and two ironworkers -- but no one was injured in the incident, according to Goodwin.

"This has been a remarkable project for the safety of the workers," he said. "We're very pleased that nobody was injured here."

The 99-by-88-foot barge is disabled and will have to be towed away, but remained under the bridge as of late Thursday afternoon, Goodwin said.

The U.S. Coast Guard responded to the incident with a boat and helicopter to make sure there was no one in the water, Coast Guard Lt. Erin Gilson said.

The Coast Guard said the crane has been secured on site Thursday night and the beam and damaged barge were taken to Oakland's pier 7.

The crash happened within federal Occupational Safety Health Administration jurisdiction.

“There are potentially significant safety issues both for the operator of the cranes and any of the workers who may be in close proximity to those cranes,” said Deanne Amaden, spokeswoman with United States Department of Labor

Officials said the accident will not affect the bridge's Labor Day opening.

The federal OSHA investigation could take up to six months to determine whether there will be any penalties or fines.