PAULSBORO, N.J. — A New Jersey bridge where a train derailed last week, releasing a hazardous chemical into the air, had a series of rail alignment problems leading up to the derailment, the National Transportation Safety Board’s top official said Monday.

Some of the problems were reported the day before Friday’s derailment.

NTSB chairman Deborah Hersman said her agency has a lot more work to do before determining the cause of the accident. ‘‘Nothing has been ruled out,’’ Hersman said.

And one important part of the investigation — a thorough inspection of the bridge and the derailed cars — must wait until crews can remove all the hazardous vinyl chloride from the area.

That work, too, paused Monday when vinyl chloride detection in the air nearby reached an unsafe level of more than one part per million around 6 a.m. Officials told residents of Paulsboro to shelter in place with their windows closed, canceled school, and pulled workers off the site of the accident.

There was an all-clear to go back outside around 11 a.m. and workers were immediately sent back to work on the site where the main priority is removing remaining chemicals from the ruptured tank car.