HOUSTON - One of the nation's largest mobile cranes collapsed at a Houston oil refinery Friday, killing four workers and injuring seven others in the latest of several fatal accidents that have raised concerns about the safety of construction cranes.

The 30-story-tall crane, capable of lifting 1 million pounds, fell over at a LyondellBasell refinery in southeast Houston about 2 p.m., said Jim Roecker, the company's vice president for refining.

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.

The casualties were in the area of the crane, but officials weren't immediately certain whether they were on the crane or under it, Roecker said.

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

The crane belonged to Deep South Crane & Rigging. Roecker described it as one of the nation's largest mobile cranes, at 300 feet tall with a 400-foot boom. Construction cranes run taller, but they are not mobile.

The crane had not been scheduled to do any work until next week, but Roecker said its engine was idling after it hit the ground.

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

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."

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, gesturing to 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.

Get $150+ in coupons in every Sunday N&O. Click here for convenient home delivery.