Ottawa, Ontario (CNN) -- Frantic cries from passengers didn't prevent an Ottawa double-decker bus from plowing into a moving train on Wednesday, a horrific morning crash that left six dead and at least 34 others injured.
Hours later, investigators offered few insights, including whether the bus driver applied brakes, if the crossing signal worked properly and, more generally, what caused the collision.
Yet some who had been on the bus, like Rebecca Guilbeault, painted a jarring picture of the moments before, during and immediately after the crash, which ended with people thrown through the air as the vehicle's front few rows sheered off.
"Everyone shouted, 'Stop! Stop! Stop!,' and then, as I looked up the bus (hit) the gate," an emotional Guilbeault told reporters as she held her young son, echoing other passengers on the bus. "(It) all impacted at once. ... Everyone flew, and there was dust everywhere."
While the VIA passenger train -- which was heading from the Canadian capital to Toronto -- partly derailed, there were no reports of injuries to those aboard.
But it was a different, gruesome story aboard the ill-fated bus, which OC Transpo chief John Manconi said can carry 90 passengers. He did not know how many were on it at the time of the crash.
Five people were pronounced dead at the scene, with another dying at a hospital, according to Ottawa paramedics Chief Anthony Di Monte.
As of Wednesday night, Ottawa police had released the identity of only one of those killed: David Woodard, a 45-year-old native of the Canadian capital, who was driving the OC Transpo bus when it crashed.
Di Monte said authorities initially assessed 11 bus passengers in critical condition, but he added that the number fell later. He did not give a firm figure, though several underwent surgery. Community members held a candlelight vigil Wednesday night to remember the victims and those struggling to recover physically and emotionally from the ordeal.
Ottawa Mayor Jim Watson said the crash's sudden nature makes it especially difficult to fathom.
"We've lost six of our neighbors, people who started off this bright, sunny day as we all did -- heading from their homes and loved ones to go about their daily lives," he said. "And then this terrible tragedy struck."
The collision occurred around 8:50 a.m. as the VIA train -- part of Canada's national railway system -- slowed while approaching its next stop in the Barrhaven neighborhood, about 10 miles from downtown Ottawa.
Heather Hogan, 26, of Kingston, Ontario, was waiting for that train on the platform when she heard "a thud, a loud metal-on-metal screech."
"Everyone said, 'What just happened?' recalled Hogan, who was about 500 feet from the crash site. "Everyone was just standing there in shock."
Mark Cogan told CTV he was driving nearby when he saw the bus "going and going and going" before it struck the train, knocking it off its rails and setting off "complete mayhem."
"There was people coming out of the windows and stuff," Cogan recalled of what was left of the bus, before he parked and ran over. "It was just a pretty devastating scene."
Within six minutes, the first of what would be 19 ambulances converged on a scene Hogan described as frantic.
First responders found "bodies and debris pretty much everywhere at the impact site," Ottawa Fire Service's Marc Messier told CTV.
They were eventually followed by members of Canada's Transportation Safety Board, which is leading the investigation.
In addition to interviewing witnesses, investigators will look at event recorders on the train, GPS data from the bus and details that might indicate if the crossing gates and signals were working properly.
Officials said Wednesday there had been no known crashes at the crossing since it opened in 2005.
Jean Laporte, the safety board head, estimated the probe could take months to complete, though authorities will be notified promptly if it's determined any public safety changes are needed.
"Our job is to determine what happened and why," Laporte told reporters on Wednesday, "with the aim of ensuring that it does not happen again."
CNN's Greg Botelho reported and wrote this story from Atlanta, and Paula Newton reported from Ottawa. CNN's Joe Sterling contributed to this report.