A home-made bomb exploded on a packed rush-hour commuter train in London injuring 22 people on Friday, police said, in what was being treated as the fifth terrorism attack in Britain this year.

Passengers on board a train heading into the capital fled as fire engulfed a carriage at Parsons Green underground station in West London after the explosion at 8.20 a.m. (0720 GMT).

Some suffered burns while others were injured in a stampede to escape. The National Health Service said 22 people had been taken to London hospitals. None were thought to be in a serious condition, the ambulance service said.

“We now assess that this was a detonation of an improvised explosive device,” Britain’s top counter-terrorism officer Mark Rowley told reporters. He said most of the injuries were thought to be flash burns.

Rowley declined to answer whether the authorities knew who was responsible or if the suspected bomber had been on the train, saying it was a live investigation which was being assisted by the intelligence services.

Pictures taken at the scene showed a white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket was in flames and there appeared to be wires coming out of the top.

“I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way toward me,” Ola Fayankinnu, who was on the train, told Reuters.

“There were phones, hats, bags all over the place and when I looked back I saw a bag with flames.”