A homemade bomb sent a scorching cloud of smoke and flames through a London subway car Friday, injuring at least 22 rush-hour commuters and sending people scrambling for safety in what police called a terrorist incident. There was no immediate claim of responsibility, and authorities gave no details on possible suspects. But the incident was quickly labeled a terrorist strike, and security measures were tightened across London’s vast mass transit network.

The British Broadcasting Corp. reported that the device had a timer, suggesting that some degree of bombmaking knowledge was used in creating it. “This was a device intended to cause significant harm,” said Prime Minister Theresa May, but it remained unclear whether the explosive detonated prematurely or malfunctioned in some way at the Parsons Green station, about three miles southwest of central London. It also was not certain if the suspected bomber was among those hurt or on the run. But in a sign that a manhunt could be mobilized, London police appealed for cellphone images from the scene.