Federal investigators have arrived at the scene of Friday's deadly crane collapse.

The fall of the 300-foot crane killed four workers and injured seven others at a Houston oil refinery. The mobile equipment apparently fell with enough force to bounce a nearby worker off the ground. It's one of the nation's largest, capable of lifting 1 million pounds.

Two severely injured workers were still being treated Saturday at Memorial Hermann-Texas Medical Center hospital. Their injuries are not reported to be life-threatening. The other injured workers have been treated and released.

A company spokesman for the LyondellBasell refinery says the crane fell during maintenance.

Federal statistics show Texas led the nation with 26 crane-related fatalities in 2005 and 2006. Cranes in Texas operate without state or local oversight. Supervision is left to federal regulators.