A home-made bomb on a packed rush-hour commuter train in London engulfed a carriage in flames and injured 22 people on Friday in Britain’s fifth major terrorism incident this year, but apparently failed to fully explode.

Passengers heading into the British capital fled in panic after the blast as the train was about to depart Parsons Green underground station in West London at 8.20 a.m. (0720 GMT).

Some suffered burns and others were injured in a stampede to escape but health officials said none were thought to be in a serious condition.

“We now assess that this was a detonation of an improvised explosive device,” Britain’s top counter-terrorism officer Mark Rowley told reporters.

Police said a hunt involving hundreds of detectives backed by the intelligence services was underway to find out who was responsible. Rowley declined to say if the suspected bomber had been on the train, saying it was a live investigation.

Britain’s threat level remained on its second-highest rank of severe following the “cowardly attack” intended to cause significant harm, said Prime Minister Theresa May. But she added the threat level would be kept under review.

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