Police in Manchester, England, have confirmed that Salman Abedi didn’t act alone on his suicide mission that killed 22 people at an Ariana Grande concert on Monday.

“It is very clear that it is a network we are investigating,” Manchester police said on Wednesday.

Authorities are in a “frantic” race against time to identify and disband the terror network to which Abedi belonged. Searches are being conducted in several areas of Britain, and four individuals have been detained, with a fifth being investigated for a suspicious package. It is thought that someone else built Abedi’s suicide vest.

Among those being detained, according to The Daily Mail, are Abedi’s younger brother, Hashem, who was arrested in Tripoli, Libya, and his father, Ramadan, who was reportedly taken into custody by masked gunmen while recording an interview in Libya. A third brother, Ismail, is already in custody in England.

The Daily Mail also reports: “Two people who knew Salman Abedi are said to have called the police counter-terrorism hotline five years ago to raise concerns that he thought ‘being a suicide bomber was OK’.”