MANCHESTER — Salman Abedi, the Manchester Arena suicide bomber, was repeatedly flagged to the authorities for his extremist views, but was not stopped by security officers.

Counter-terrorism agencies were facing questions after it emerged that Abedi told friends that “being a suicide bomber was okay”, prompting them to call the government’s anti-terrorism hotline.

Sources suggest that the authorities were informed of the danger posed by Abedi, 22, on at least five separate occasions in the five years prior to the attack on Monday night.

The authorities were also aware that Abedi’s father was linked to a well-known militant Islamist group in Libya, which is proscribed in Britain. Abedi also had links to several British-based jihadists with Islamic State (IS) connections.

On Wednesday, his father was detained by Libyan militia in the capital Tripoli while the suicide bomber’s two brothers have separately been arrested on suspicion of terrorism offences.

Home Secretary Amber Rudd conceded on Wednesday night that Abedi was known the intelligence services.

The apparent lapses emerged on a day of heightened police activity as the hunt for Abedi’s terror cell intensified.

Early yesterday, police said they conducted a controlled explosion in the south of Manchester, where they were carrying out searches in the Moss Side area connected to the attack.

Underscoring jitters in the city, bomb disposal units were rushed to a college in Manchester, which later turned out to be a false alarm.

As the nation mourned, Queen Elizabeth II visited children injured in the attack at a hospital in Manchester.

“It’s dreadful. Very wicked to target that sort of thing,” she told Evie Mills, 14, and her parents.

The attack killed 22 people and injured more than 100.

Twelve of the injured are aged under 16 and one of those killed was an eight-year-old girl.

The Telegraph spoke to a community leader who said that Abedi, a university dropout, was reported two years ago “because he thought he was involved in extremism and terrorism”.

Mr Mohammed Shafiq, chief executive of the Ramadhan Foundation, said: “People in the community expressed concerns about the way this man was behaving and reported it in the right way using the right channels. They did not hear anything since.”

Two friends of Abedi also became so worried they separately telephoned the police counter-terrorism hotline five years ago and again last year.

Mr Akram Ramadan, 49, part of the close-knit Libyan community in south Manchester, said Abedi had been banned from Didsbury Mosque after he had confronted the Imam who was delivering an anti-extremist sermon.

Mr Ramadan said he understood that Abedi had been placed on a “watch list” because the mosque reported him to the authorities for his extremist views.

Abedi’s own family background might also have been a red flag to authorities. His father was a member of the militant Libyan Islamic Fighting Group.

Yet Abedi was able to travel frequently between the UK and Libya, where it is believed he trained in bombmaking and possibly travelled to Syria.

His youngest brother, Hisham — who is photographed on social media wielding an automatic rifle — was arrested late Wednesday night by Libyan authorities, who suspect him of knowing about the Manchester plot in advance and plotting his own attack in Tripoli.

A spokesman for the Deterrence Force, which acts as Libya’s Government of National Accord’s police, said the brother was aware of Abedi’s plan and the siblings were both members of IS.

It also said on its Facebook page that Hisham had also been “under surveillance for a month and a half”. A relative told AFP that Abedi had travelled to Manchester from Libya four days before the bombing.

A spokesman for the Libyan authorities said Abedi had made a call to his mother to “say goodbye” before committing the atrocity. AGENCIES