An investigation into an accident that toppled a crane working on the Bay Bridge Thursday is expected to take around a month but shouldn't delay construction or the new bridge's opening, officials said Friday.

At least one of the cranes involved in yesterday's accident was towed back to the Bay Bridge construction campus Friday as the investigation into Thursday's accident gets underway, Bay Bridge spokesman Andrew Gordon said.

It remains unclear exactly which agencies might be involved, but the investigation is expected to take around a month, he said.

Two cranes supported on barges were working in tandem to lower a 129-ton section of the temporary support structure for the bridge's new suspension span via a large basket onto a barge when the accident occurred, Metropolitan Transportation Commission spokesman John Goodwin said Thursday.

For reasons that have yet to be determined, at about 12:15 p.m., the 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 99- by 88-foot 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.

The reconstructed and seismically upgraded Bay Bridge is scheduled to open Labor Day weekend with a grand multi-day celebration.

Officials said today that the crane accident will not slow construction, since it halts only the removal of temporary support structures for the 2,047-foot suspension span east of Yerba Buena Island. The span is already self-supporting.