Officials launched a dragnet for the terrorist who detonated a homemade bucket bomb on a packed London subway train during Friday morning rush hour, injuring at least 22 people.

Police quickly called the incident on the District Line train at Parsons Green station a “terrorist incident.” It was the fifth terrorist attack in Britain this year. Police said they have not arrested anyone in connection to the bombing but hundreds of detectives were trying to hunt down the perpetrator

The Metropolitan Police force said police “are making fast-time inquiries to establish who was responsible and are working closely with the security services.”

Counterterrorism policing chief Mark Rowley said hundreds of detectives were scanning surveillance camera footage, carrying out forensic work and speaking to witnesses.

Scotland Yard said an improvised explosive device was used in the attack, and London’s Metropolitan Police counter-terrorism unit was also investigating; however, police did not immediately provide any details on any suspects.