The attack on a Manchester pop concert that killed 22 people was “likely” the work of more than one person, British Interior Minister Amber Rudd said Wednesday, revealing that suicide bomber Salman Abedi was known “up to a point” to the British intelligence services and police.

According to Rudd, up to 3,800 soldiers will be deployed on the streets of the UK in response to the worst militant attack for over a decade.

“It was a devastating occasion, it was more sophisticated than some of the attacks we’ve seen before, and it seems likely — possible — that he wasn’t doing this on his own,” she told BBC radio.

She confirmed that Abedi, a British man of Libyan heritage who died in the explosion on Monday night, had been on the radar of the security services.

“We do know that he was known up to a point to the intelligence services,” she told Sky News.

Clarifying this on the BBC she said: “The security services will know a lot of people. It doesn’t mean that they’re expected to arrest everybody they know.

“But it is somebody that they had known.”

The interior minister also believed the suicide bomber at an Ariana Grande concert in Manchester, had recently returned from Libya.

Asked to confirm whether Abedi had recently come back from the country, Rudd told BBC: “Yes, I believe that has been confirmed. When this operation is over, we will want to look at his background and what happened, how he became radicalized and what support he might have been given.”

The minister said she was “not surprised at all” that the attack had been claimed by ISIS, but said there was no information yet to confirm the extremist organization’s active direction.

French Interior Minister Gerard Collomb also said the suicide bomber is believed to have traveled to Syria and had “proven” links with ISIS.

Collomb said on BFM television that British and French intelligence have information that Abedi had been to Syria. He did not provide details, and said it is unclear whether Abedi was part of a larger network of attackers.

London police said Wednesday they would be calling in the army to help guard key landmarks, including Buckingham Palace, the Houses of Parliament and foreign embassies.

Britain’s national terror threat level was raised late Tuesday to “critical”, meaning another attack may be imminent, following the concert bombing.

Rudd said she had “complete confidence” in the security services.

British police on Wednesday made three more arrests over the Manchester attack, bringing the number of men in custody to four.

“Three men have been arrested after police executed warrants in South Manchester, in connection with the ongoing investigation into Monday night’s horrific attack at the Manchester arena,” a statement said.

Also Wednesday, Britain’s parliament said it would close to the public with immediate effect due to an increased security threat after the suicide bombing.

“All tours, events and banqueting will be cancelled with immediate effect. This arrangement will remain in place until the advice changes,” the parliament said on its website.

Parliament is not currently sitting, due to a national election taking place on June 8.