(CNN) -- A railroad swing bridge in New Jersey that collapsed near the Delaware River was inspected in November, the chairwoman of the National Transportation Safety Board said Sunday.
Several cars on a train that was passing over the bridge derailed Friday morning. Some fell into the water below and one car hit another, tearing it open and releasing a vapor cloud from hazardous materials, Debbie Hersman told reporters at a new conference.
Investigators are just beginning to piece together information from the accident and whether the bridge collapse or the train derailment occurred first.
The bridge is subject to several different types of inspections, a biannual structure assessment that occurred in May and November, and a quarterly mechanical examination. Inspectors also look at the parts of the bridge underwater once every five years. The next such inspection was scheduled for September 2014.
Hersman said the track is inspected often, most recently on November 20.
NTSB investigators have interviewed train crews that recently used the bridge and some of them said there have been signal issues with the bridge, which swings across the river when trains need to cross the waterway. Herman said a team from Conrail, which owns and operates the track and the bridge, was at the structure on Thursday.
On Friday, the train engineer tried several times to get the bridge signal to turn to green using a keypad, but it stayed red.
After about six minutes, the train crew spoke with the dispatcher who monitors the bridge and was given the OK to cross.
The front of the train reached the other side of the bridge when the crew said they saw the bridge collapse. Emergency brakes activated, but by that time seven of the cars had derailed, four of which fell in the water.
One of the cars in the water was carrying ethanol. The three others were carrying vinyl chloride, and the crash tore open a 1-by-3-foot hole in one of them.
The bridge near Paulsboro collapsed in 2009 and was rebuilt, Hersman said.
Saturday morning, crews found slightly elevated levels of fumes in the immediate area, but still well below acceptable thresholds, Coast Guard Capt. Cathy Moore said.
Twelve square blocks near the scene -- approximately 48 households -- were evacuated Friday. The residents have been told to stay away for three days, Moore said.
CNN affiliate WPVI reported that 70 people went to emergency rooms, but no one was injured.
Hersman said NTSB investigators are waiting to be told it's safe to conduct on-site information gathering.
Her teams will pay close attention to the locking mechanisms on the bridge, she said Sunday.
The NTSB has examined most of the cars didn't derail and found no problems with them, Hersman said.
Investigators will also be looking at the possibility that elevated levels of water created by Hurricane Sandy affected the bridge. The structure crosses a creek that flows into the Delaware River. Water levels in the creek also rise and fall with the tides.