Pictures from a camera mounted on the front of a derailed freight train show that the train went off the tracks after it struck a garbage truck that crossed the tracks while the train's horn was blasting warnings.
Fifteen cars of the CSX train crashed to the ground in an industrial area of White Marsh, Md., on Tuesday. The derailment caused an explosion so strong that several nearby buildings collapsed or were damaged.
An overturned garbage truck could be seen at the scene and the driver was injured, according to the Baltimore County Police and Fire Department.
The truck driver has been identified as John Alban Jr., 50, a retired Baltimore County firefighter and current volunteer firefighter for the Hyde Park Volunteer Fire Department. Alban was taken to a hospital where he is in serious but stable condition, authorities said.
Inspectors from the National Transportation Safety Board viewed the video from the train's camera as well as surveillance recordings from a nearby business.
"Using those pieces of data, I'll paint a picture for you," said NTSB board member Robert Sumwalt at a news conference today.
"Seventeen seconds before collision, [the] train horn blasts begin... We are able to hear the three blasts going up to the point of impact," Sumwalt said.
"Five seconds prior to the collision, we see the front of the truck coming into the picture. Three seconds prior to the impact, the front of the truck enters the track and at impact the train strikes the right rear tandem axel of the truck," he said.
A camera from a nearby business captured video of the first hints of smoke 33 seconds after the impact.
Sumwalt said the train, which had been traveling at 49 mph before the crash, traveled more than 4,000 feet after its emergency brakes were applied.
Despite the video evidence, Sumwalt said, "We will not be determining cause while we're on the scene and we will not speculate about what may have caused the accident."
He said the crossing had stop signs on each side, but did not have gates, lights or bells.
CSX official Kristin Seay said earlier that "Four cars contained terephthalic acid. One car contained fluorosilicic acid. That rail car was empty, but CSX lists it as a 'residue car' because the car may still contain traces of the product."
Long-term exposure to terephthalic acid can cause respiratory problems, authorities said, citing a state of New Jersey report.
Contributing reporting by ABC News' Howard Price.