A Friday morning explosion at the Parson’s Green London Underground train station caused panic and left at least 22 people injured in a bombing attack, but luckily caused no fatalities.

It quickly became clear that the attack could have been much worse, as the “improvised explosive device” used appeared to have failed, at least partially.

“Deputy Assistant Commissioner Neil Basu, the senior national coordinator for CT (counter-terrorism) policing, has declared it a terrorist incident,” the Metropolitan Police said in a statement about two hours after chaos erupted on a train at Parsons Green station, in the southwest part of the British capital.

Commuters from the train posted photos on social media showing police and firefighters on the scene and describing panic as people escaped the train car and then got out of the station, which is an above-ground stop on the London Underground network, known as the tube.

London Ambulance Service said 18 people were taken to area hospitals with injuries, but that “none are thought to be in a serious or life-threatening condition.” Four other people made it to London hospitals on their own with injuries from the attack.

Many of the injuries appeared to be “flash burns,” according to the London police. Others were reportedly hurt in the crush as hundreds of commuters rushed to get out of the station.

Peter Crowly, who was on the train, told BBC News shortly after the incident that he “heard a large bang from other side of the tube train” and that a “firewall” flashed above his head. He posted photos on his Twitter account showing a burn to his forehead and a patch of hair that appeared to be scorched.

Multiple photos posted online showed what appeared to be a large plastic bucket sitting inside a plastic shopping bag, with what looked like charring around the top of the bucket.

The photos showed no obvious signs of an explosion, but some images showed small flames still burning around the package.

Other witnesses from inside the train reported seeing a large flash or fireball. There was no extensive damage seen around the bag containing the charred bucket. Some images appeared to show wires protruding from the top of the bag.

Police would not immediately confirm that the package was connected to the explosion. An officer at the scene described an “item exploding to a small degree” on the train.

Several hours later, Metropolitan Police Assistant Commissioner Mark Rowley said it was clear there had been a “detonation of an improvised explosive device” on the train.

He would only say police were chasing down “many urgent enquires,” including looking at security camera video from the area.

Rowley would not confirm explicitly that police were searching for any specific suspects in the attack. He said Britain’s MI5 domestic spy agency was helping with what he called, “very much a live investigation.” London Mayor Sadiq Kahn did acknowledge, however, in multiple interviews, that a manhunt was underway.

A security source with knowledge of that investigation told CBS News that, according to initial assessments, the bucket likely contained a homemade explosive consistent with use in recent terrorism incidents.

The source said the homemade bomb appeared to be fitted with a timer, raising the question of whether the perpetrator was even on the train when it detonated. Loose screws were also found near the device thought to be intended for increased fragmentation, to cause more damage.

Richard Walton, former head of counter-terrorism at the Metropolitan Police and now a CBS News security consultant, said investigators would be trying to quickly determine if the small blast was caused by a viable explosive device.

If it was a viable bomb, a former bomb disposal officer tells CBS News it’s possible that the bang and fire were caused by a detonator exploding, but failing to ignite the primary explosive charge.

London Underground suspended services on a stretch of the District Line, the one affected by the incident, but said the rest of the network would continue running.

The London Fire Brigade confirmed in a tweet that firefighters had responded to a call at 8:21 a.m. (3:21 a.m. Eastern), during London’s morning rush hour, at the busy station in the Fulham area.

A hazardous area response team was on the scene from the London Ambulance Service, along with dozens of firefighters, and armed Metropolitan Police officers could also be seen around the station, which was sealed off.

British Prime Minister Theresa May was “receiving regular updates” on the situation at Parsons Green, according to her office.

London Mayor Sadiq Khan said in a statement that he would join May at a meeting later Friday of the national government’s “COBRA” emergency response team.