( 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 did n'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 .
Board chairwoman said bridge is inspected for structural problems twice a year . Engineer still waiting for all clear to go on site . Train derailment led to one car being overturned , releasing hazardous fumes . It occurred after train stop n't get green light to cross bridge , got OK from dispatcher .