A British government spokesperson said Monday that the agency had opened two internal investigations last week, amid reports that the British authorities had been alerted to Abedi’s extremist views at least three times before the bombing.
The 22-year-old bomber, a Manchester resident of Libyan descent, had previously been flagged as a “person of interest” by MI5, said a law enforcement official, who would speak only anonymously about details of an active investigation. But he had been deemed not dangerous, after which his file was closed and his name was taken off a list of 3,000 people under active investigation.
When a file is closed, all security alerts are removed. As a result, no warnings would have come up on Abedi’s passport when he went through border controls, the official said.
Several people who knew Abedi, including some of his friends, said that they had warned authorities repeatedly about his radical views over the past five years.
He was banned from Didsbury mosque, where his family worshipped, after he shouted at and gave a threatening stare to an imam who had delivered a sermon condemning the ideology of Daesh, also known as ISIS or ISIL, according to Akram Ramadan, a member of the local Libyan community.
At least two congregants from the mosque reported Abedi to the authorities two years ago, the law enforcement official confirmed.
It is highly unusual for the British authorities to make public internal investigations into possible security lapses, but Britain’s home secretary, Amber Rudd, welcomed the MI5 review Monday, saying it was “ the right first step” in learning lessons from last week’s attack.
“There is a lot of information coming out at the moment about what happened, how this occurred, what people might or might not have known,” Rudd said in a television interview with Sky News on Monday. “And I think it is right that the MI5 takes a look to find out what the facts are.”
She stressed that, while the investigation into the possible security failure would be useful, the focus should now be on the still quite active terrorism investigation.
Detectives investigating the attack said Friday that they had apprehended most of the members of the network believed to have assisted Abedi on his suicide mission.
Police carried out a series of armed raids at addresses across Greater Manchester over the weekend, which ended with the arrest of a 25-year-old man in the Old Trafford area. The operation expanded Monday to Sussex, on the southeast coast of England, where counterterrorism police arrested a 23-year-old.
That brought the total number of arrests to 16, Greater Manchester Police said in a statement Monday. Two of the detainees have been released without charge.
Investigators believe that Abedi received extensive terrorist training in Libya before returning to Britain, where he was assisted by a local network for several days before his attack. The bombing, at Manchester Arena at the end of an Ariana Grande concert on May 22, was Britain’s deadliest since 2005.
“The suspect would have received training abroad, without a shadow of a doubt,” said David Videcette, a former Metropolitan Police detective. “To acquire these skills you can’t sit in your bedroom and watch a YouTube video. It takes practice.”
Upon initial analysis of the attack, Videcette said it pointed to a sophisticated attack cell that would have supplied logistical, technical and emotional support to the bomber “in order to keep him in a place where he is willing to blow himself up.”
While the police have said that most of Abedi’s network has been captured, Rudd warned Sunday that some of its members could still be at large. The government has, however, scaled down its national terrorist threat level to “severe” — the second-highest level — from “critical.”