LONDON (AP) — Manchester’s police chief has told reporters that it is clear “this is a network we are investigating” as he gave an update on the probe into the bomb attack at a pop concert in the city.

Chief Constable Ian Hopkins said Wednesday that police are carrying out extensive searches across Manchester as part of their probe.

Hopkins declined to comment on whether police have found the alleged maker of the explosive device used in Monday night’s attack.

His comments followed media reports that the alleged bomber, Salman Abedi, acted as a “mule” for others.

Hopkins says a serving police officer was among the 22 people confirmed killed in the attack.

He confirmed that a total of four suspects have been detained so far.