MI5 is to hold an inquiry into the way it dealt with warnings from the public that the Manchester suicide bomber, Salman Abedi, was a potential threat.
The security service, which was alerted to Abedi’s extremist views three times prior to last Monday’s attack, will examine how it dealt with the warnings.
Home Secretary Amber Rudd said it was right for MI5 to review its processes.
Early on Monday a man, 23, was arrested in Shoreham-by-Sea, West Sussex, on suspicion of terrorism offences.
Greater Manchester Police said they had also executed search warrants at addresses in the Whalley Range area of Manchester and in Chester overnight.
Seven children were among the 22 people who died when Abedi detonated a bomb on 22 May at the end of a concert by US singer Ariana Grande at Manchester Arena.
In all, 14 men are now being questioned in connection with the investigation into the attack.
On Sunday, two men were arrested in Manchester – a 19-year-old man in Gorton and a 25-year-old man in the Old Trafford area.
MI5 has launched a “post incident investigation” into how the Manchester bomber was overlooked, while a separate report is being prepared for ministers and those who oversee the work of the service.
A Whitehall official said previously that Abedi was one of a “pool” of former subjects of interest whose risk remained “subject to review” by the security service and its partners.
Born in Manchester to Libyan parents. BBC Newsnight reported that when Abedi was 16 he fought against the Colonel Gaddafi regime with his father during the school holidays.
It was while at Manchester College that two people who knew Abedi have confirmed they made separate calls to an anti-terrorism hotline to warn the police about his extremist views.
Greater Manchester Police have made an appeal to the public for information about Abedi’s movements since 18 May, when he returned to the UK from abroad.
The home secretary would not comment specifically on what the security service knew about Abedi, but insisted it was right for MI5 to review its processes because “the threat is always changing”.
The Times is reporting that a “key” tool which gives law enforcement the power to stop Britons suspected of involvement in terrorist activity abroad at the UK border has only been used once it was introduced in 2015.
Ms Rudd said it was not for her to demand that a certain number of Temporary Exclusion Orders be used, but to make sure the security service has the “tools they can use” to protect the public.
“We need to put our efforts into protecting young people,” she told BBC News. “People in our communities who are being targeted by Daesh [the Islamic State terror group] and fed a false ideology.”
She said the government had “never understated” the threat of a terror attack in the UK, adding: “We have to level with people.”
Reality Check: How can police stop terror suspects travelling?
Schedule 7 of the Terrorism Act 2000 gives the police the power to stop, search and hold individuals at ports, airports and international railway stations.
Initially individuals could be questioned for up to nine hours – although a change in the guidelines means it is now up to six hours. The police also have the power to inspect electronic devices such as phones and laptops.
The police also have the power to seize travel documents for up to 14 days if they believe someone is travelling in order to engage in terrorist activity.
Another option is a Temporary Exclusion Order (TEO), which can be used against British citizens suspected of involvement in terrorist activity abroad. They are designed to stop suspects from re-entering the UK unless they give themselves up at the border.