The United Kingdom Police on Wednesday arrested three more men in connection with the attack at an Ariana Grande concert in Manchester. The brother of the suicide bomber, who had been identified as a Libyan named Salman Abedi, was arrested on Tuesday, reported BBC. With this, four men are now in custody.

“I think it’s very clear that this is a network we are investigating,” said Manchester Police Chief Constable Ian Hopkins said after the arrests, The Guardian reported. “Our priority, along with the police counter-terrorism network and our security partners, is to continue to establish whether he was acting alone or working as part of a wider network,” Hopkins had said earlier in the day.

Meanwhile, the British government has raised the terror threat level from “severe” to “critical”. Prime Minister Theresa May had said the “critical” level of a terror threat meant further attacks might be imminent. “ May also said security at key sites or public events will be increased and asked the public not to be “unduly alarmed”. Military personnel may also be deployed at public events like concerts or sports events, she added.

According to reports, Abedi had used a device built by someone else. Interior Minister Amber Rudd also said Abedi had not acted alone, reported Reuters.

The explosion, which killed 22 people and injured 59 others, took place at an arena where American singer Ariana Grande was performing. The majority of the audience consisted of teenage fans. The Islamic State had claimed responsibility for the attack on Monday via a messaging app, Telegram.