Houston (AP) -  The largest mobile crane in the nation collapsed at a Houston refinery Friday, killing four contract workers and injuring six others, a company vice president said.

The crane, capable of lifting 800,000 pounds, fell over at a LyondellBasell refinery in southeast Houston about 2 p.m. The casualties were in the area of the crane, but officials still aren't certain whether they were on the crane or under it, said John Roecker, the company's vice president for refining.

Three of the injured were treated and released at the scene, said Houston Fire Department Assistant Chief Omero Longoria. Two severely injured workers were taken by helicopter to Memorial Hermann-Texas Medical Center hospital and the other injured worker was taken to a hospital by ambulance.

The crane, whose exact dimensions were not immediately available, belonged to Deep South Crane & Rigging. It was delivered in pieces and assembled on site about a month ago. It was brought in to remove large drums from inside a coking unit whose roof had been cut off to allow the crane access, Roecker said.

But the crane was not scheduled to do any work Friday. Roecker said, however, that the crane's engine was idling after it hit the ground.

Roecker said he'd seen the crane lift 800,000 pounds during a test run last week, and described it as the nation's largest mobile crane.

Deep South spokeswoman Margaret Landry issued a statement from the company's headquarters in Baton Rouge, La., saying it was investigating "to determine the root cause, correct it and ensure that this type of tragedy does not occur again."

The large crane fell on or knocked a smaller, nearby crane. A picnic table under a tent was also nearby, but authorities said they didn't know yet if the cranes hit it or whether anyone was inside.

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

The massive deep red crane lay on top of a smaller, bright yellow crane on the grounds of the refinery. Ambulances and fire trucks were lined up outside.

About 1,500 contract workers are employed at the refinery, he said.

Mattie Graham stood with her husband, Deep South worker Horace Graham, at the plant near the scene of the accident.

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

Roecker said all personnel at the plant were accounted for, and the plant was operating as usual.

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

Texas is one of 35 states that do not require crane operators to be licensed. Earlier this year in Dallas, city officials found that eight of 23 cranes being used across the city had uncertified operators at the controls.

OSHA standards require cranes to undergo annual inspections, but it is a self-policing mandate for crane owners. Federal law requires that inspection records be kept, but not submitted.

Roecker said OSHA and other regulatory agencies had been notified of Friday's accident.

The Houston refinery is one of the world's largest for processing high-sulfer crude oil. The facility itself covers about 700 acres along the Houston Ship Channel at the city limits of Houston and Pasadena.

Lyondell Chemical, a U.S. company, and the Dutch firm Basell were rivals until they announced a $12.1 billion deal last July to create one of the world's largest chemical companies.

On the chemical side, Lyondell produces ethylene, a crucial precursor to a range of other chemicals, as well as propylene oxide, which is also used in producing a variety of chemical products.

Basell focuses on polyolefins, common types of plastic.

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

"It's really odd for these cranes to have any issues because there are so many eyes looking in," Williams said.

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 past decade.

An Associated Press analysis in June found that cities and states have wildly 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.

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