(CNN) -- The crew aboard a train that derailed near Baltimore, killing two young women, did not apply the emergency brakes just before the accident, according to the National Transportation Safety Board.
The air brakes were automatically triggered, agency spokesman Jim Southworth said Wednesday, noting that a possible rupture in an air line may have caused the emergency braking.
But Southworth said the investigation was just in its beginning stages.
He added that the train -- with two locomotives and 80 cars -- had been traveling about 25 mph when it jumped the tracks.
The first 21 cars behind the locomotives derailed.
Rose Mayr and Elizabeth Nass, both 19, were killed early Tuesday when the train derailed on the bridge on which they were sitting, spilling coal and burying them, police said.
The derailed train also damaged fiber-optic lines and temporarily disrupted Internet service at the U.S. naval base at Guantanamo Bay, Cuba, delaying the trial for Khalid Sheikh Mohammed and four others facing terrorism charges this week, a base spokesman said.
The facility lost about 50% of its connectivity, which is serviced by satellite downlink locations in Maine and Maryland, according to Capt. Robert Durand.
The service was restored withing 24 hours, a spokesman for Verizon, which maintains the line, said.
Howard County Executive Ken Ulman said that once the initial investigation has been completed, authorities will assess how Mayr and Nass got access to the bridge.
They were apparently sitting on the bridge ledge in Ellicott City, Maryland, just after midnight with their backs to the train when it derailed, according to a written statement from Howard County police.
The two posted photos to Twitter shortly before the crash. One showed feet dangling over a road, with the caption "Levitating." Another appeared to look down Main Street.
Nass tweeted, "Drinking on top of the Ellicott City sign with @ r0se_petals." The name Ellicott City is painted on the railroad bridge.
Mayr and Nass were high school classmates of David Butko's. He said he grew concerned when he heard about the accident and saw the pictures on Twitter that indicated Mayr and Nass were on the bridge.
"We went to the site ... hung around, but the police wouldn't let us in," he said.
Nass was supposed to go back to college at James Madison in Virginia on Tuesday, he said. Classes begin in six days, according the university's website. A spokesman said Nass was an honors student in the interdisciplinary liberal studies program.
Mayr was a nursing student at the University of Delaware who was about to start her junior year.
"They were both terrific kids," Mount Hebron High School Principal Scott Ruehl said. "Both girls were really involved in the dance program. They were pleasant; they always said hi in the hallway."
Butko said Mayr was a very funny girl.
"She always had a smile and a joke," he said. "She was goofy at times, but could be there for you when you needed it."
Marmie Edwards, a spokeswoman for Operation Lifesaver, a nonprofit organization that promotes railroad safety, said the number of fatalities in 2012 involving people on tracks was up significantly.
"It's probably just as well to stay away, not just to be off the tracks, but stay away from the train so you know you're going to be safe," she said. "Because it's hard to say, 'OK, here it's safe; there it's not.' So just stay away."
According to the Federal Railroad Administration, in the first five months of the year, 178 people were killed in accidents on or near tracks, not including at a railroad crossings.
CNN's Brian Todd and Dugald McConnell contributed to this report.