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 still aren’t certain whether they were on the crane or under it, Roecker said.

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.