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. The casualties were in the area of the crane, but officials still weren’t certain whether they were on the crane or under it, said John Roecker, the company’s vice president for refining.

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