It's not clear how long it will take workers to clear a hazardous gas from a ruptured tank car that derailed from a freight train in southern New Jersey.

More than 100 Paulsboro residents won't be able to return to their homes in the 12-block evacuation zone until at least Saturday.

Officials must remove the vinyl chloride before they can clear the track and examine what caused Friday's derailment.

Federal investigators said Sunday that Conrail crews had studied reported signal problems at the railroad bridge where the derailment occurred.

The National Transportation Safety Board is reviewing records with a focus on both the signal problems reported recently and a 2009 train derailment on the same bridge.

The derailment sent dozens to a hospital, but no serious injuries have been reported.