Hitting the ground with enough force to bounce a nearby worker off the ground, one of the nation's largest mobile cranes collapsed at a Houston oil refinery, killing four workers and injuring seven others.

Investigators for the Occupational Safety and Health Administration were at the site Saturday, officials of the refinery and the crane company said.

Deep South Crane & Rigging, the Baton Rouge, La., company that owns the 300-foot-tall equipment, plans to work with the federal investigators looking into what is the latest in a series of fatal accidents involving cranes around the country.

"We will cooperate fully with all investigations that may arise from this tragic incident. We will provide information as we gather and verify it," company spokeswoman Margaret Landry said in a statement.

The crane, capable of lifting 1 million pounds, toppled at a LyondellBasell refinery in southeast Houston on Friday afternoon.

Two severely injured workers were still being treated Saturday at Memorial Hermann-Texas Medical Center hospital for injuries that LyondellBasell said were not life threatening. The other injured workers had been released after treatment.

The crane collapsed during maintenance, LyondellBasell officials said. It had not been scheduled to be used for any work until next week, but its engine was idling after it hit the ground, said Jim Roecker, the company's vice president for refining.

"This is a traumatic experience for all of us. We have to focus on the safety and health of our employees," Roecker said.

Micheal Gabriel, 22, of Spring, told reporters as he left a hospital Friday night that he was lifted off the ground by the crane's impact.

Gabriel, a contract worker, said he didn't see the crane fall. "I was in shock. I was crying. It was bad."

He told a relative that he was in a tent where workers eat lunch when he heard a loud pop and people started shouting for people to run, the Houston Chronicle reported.

Near the scene of the collapse, Mattie Graham stood with her husband, Deep South worker Horace Graham.

"I'm thinking about their families. He could have been there today," she said of her husband.

The refinery has about 3,000 LyondellBasell workers and 1,500 contract workers, Roecker said. He said all personnel at the plant were accounted for, and the plant was operating as usual.

Crane safety has been getting extra scrutiny in recent months because of an alarming number of crane-related deaths in places such as New York, Miami and Las Vegas.

In New York City, two crane accidents since March have killed nine people _ a greater number than the total deaths from cranes over the previous decade.

An Associated Press analysis in June found that cities and states have widely varying rules governing construction cranes, and some have no regulations at all, choosing instead to rely on federal guidelines dating back nearly 40 years that some experts say haven't kept up with technological advances.

Texas led the nation with 26 crane-related fatalities in 2005 and 2006, according to federal statistics. Cranes in Texas operate without any state or local oversight, leaving that job to federal regulators.

The crane at the refinery had been delivered in pieces and assembled on site within the last month. It was brought in to remove the roof of a coker unit so large drums could be removed from inside, Roecker said. Cokers convert crude oil to petroleum products.

Roecker described it as one of the nation's largest mobile cranes; construction cranes run taller, but they are not mobile.

East Texas Crane Academy president Joe Bob Williams, whose clients include Lyondell, said it's unusual for such cranes to fail because of the number of people involved in their maintenance.

Cameras are mounted around the plant and Roecker said the company hopes that video from those cameras will help it learn what happened.

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Associated Press writers Ana Ley and John Porretto and photographer David Phillip in Houston, Paul J. Weber in Dallas and researcher Judith Auesebel in New York contributed to this report.