Salman Abedi, the suicide bomber behind the deadly Manchester Arena bombing Monday night, was reportedly already known to security services but was not judged to be a threat – making Manchester the latest European city to be hit by a terror attack committed by someone known to authorities.

Authorities revealed the identity of the suspected suicide bomber Tuesday afternoon, hours after the Libyan origin British citizen was named by CBS News.

The terrorist detonated a nail bomb at an exit to the arena as people left an Ariana Grande pop concert that had just finished. Twenty-two people were killed and dozens injured, many of whom are believed to be teenage girls and children.

“This was among the worst terrorist incidents we have ever experienced in the United Kingdom,” Prime Minister Theresa May said in a statement outside 10 Downing Street Tuesday morning.

The prime minister said police and security teams were working “at speed” to get the full picture of what happened, and to establish if Abedi was working by himself or as part of a wider group. Mrs. May had earlier said they believed they knew the identity of the perpetrator, but would not confirm his name.

The Evening Standard reported the bomber, who has not immediately been identified by authorities, “was known to security services but they did not believe he posed an immediate threat”. Islamic State has claimed responsibility for the attack online.

If Abedi was indeed known to authorities, it would mark the latest in a number of terrorists who committed terrorist atrocities despite having been monitored by security services.

Khalid Masood, the Islamic terrorist who in March mowed down people with his car before stabbing a police officer to death outside the Palace of Westminster, had been monitored by MI5 over concerns of “violent extremism” but was no longer deemed to be a threat.

Anis Amri, who drove a truck through a Berlin Christmas market and killed 12 people in December, had been identified as a terrorist threat months before the attack but was not apprehended.

In France, the gunman who killed a police officer at the Champs-Elysees in April had been the subject of a counterterrorism investigation in March. Some of the attackers behind the 2015 Paris terror attack had also been monitored by both Belgian and French intelligence services – but were still able to coordinate a major terrorist attack, killing 130 people.

The last major terror attack to hit British soil, the 7/7 bombings in 2005 which left more than 50 people dead, was committed by four suicide bombers — two of whom were under surveillance by authorities, but were reportedly not fully investigated due to a “lack of resources”.

Greater Manchester Police announced Tuesday that they had arrested a 23-year-old man in South Manchester in connection with Monday’s bombing, but no further details were immediately available.

Adam Shaw is a politics reporter for Breitbart News based in New York. Follow Adam on Twitter: @AdamShawNY