The bomb was detonated during rush hour.
While London authorities investigate a rush hour explosion aboard a commuter train as a terror attack, one man is revealing his experience as he was right beside the blast.
Officials in the U.K. said Friday that the blast on the London Underground was caused by an improvised explosive device, or IED — one that is believed not to have detonated entirely.
Watch: Police Release CCTV Photos of Manchester Bomber on the Night of Terror Attack
But the explosion was enough to strike fear in commuters, including a witness who walked away with burns.
"I heard a large bang from the doors on the other side of the train. And this fireball above my head singed all my hair, there were burn marks on the top of my head," the witness said. "Everyone just ran off the train. It was scary."
Police say the man was among about 20 people who suffered some sort of injury from the blast. None of the injuries were reportedly life-threatening.
The incident, however, is being treated as a terrorist attack, one that could have taken countless lives as it occurred at the height of London's morning rush hour.
"The investigation is being led by the Metropolitan Police counter-terrorism command and part of national counter-terrorism policing network. And there are many urgent inquiries ongoing now with hundreds of detectives involved looking at CCTV, forensic work and speaking to witnesses.
"This investigation is supported by our colleagues by MI5 and bringing their intelligence expertise to bear on the case," Mark Rowley, Assistant Commissioner for Specialist Operations of the Metropolitan Police Service, told reporters.
Police did not provide details on any suspects, but U.S. President Donald Trump painted them with his signature broad strokes on Twitter.
