Nearly three months before a 22-year-old suicide bomber blew himself up at Ariana Grande's concert in Manchester Arena in the United Kingdom, the Federal Bureau of Investigation had warned its British counterpart, MI5, that Salman Abedi was planning a terror attack in the country.

- A 25-year-old man is arrested in Old Trafford, to the east of the city.

In addition, police raided a property in a southern Manchester suburb as they step up their investigation into last week's bombing that killed 22 people.

A senior Whitehall source previously revealed the mass murderer was a "former subject of interest" to the security services whose risk "remained subject to review". This is a handout photo taken from CCTV and issued on Saturday, May 27, 2017 by Greater Manchester Police who have altered the surrounding area of Salman Abedi, in an unknown location on the night of the attack.

Eleven other men, aged between 18 and 44, are in custody.

The security service will examine assumptions that were made about Abedi before the attack, and has launched a "post-incident investigation" into how the bomber was overlooked, the BBC said.

The suitcase was not used in the attack, which was carried out when Abedi detonated an improvised bomb minutes after the concert ended, Greater Manchester Police Detective Chief Superintendent Russ Jackson said. The explosion killed 22 people - including seven children under 18 - and injured more than 100.

Abedi was already placed on terror watch list in the United States after he came under the scanner in 2016 during an investigation into terror groups operating in Libya, the Independent reported.

British Home Secretary Amber Rudd also confirmed Abedi did not act alone and that he was known to security services.

In an interview with the BBC on Sunday, Rudd acknowledged some members of Abedi's network could still be free.

Police have released surveillance-camera images of Abedi on the night of the attack that show him dressed in sneakers, jeans, a dark jacket and a baseball cap.

Officers from Cheshire Police and Counter Terrorism Policing North West searched an address in Chester in connection with the attack. The country remains on high alert, however, as officials caution that some suspects who helped Abedi may still be at large.

Prime Minister Theresa May said developments in the investigation into the bombing meant that intelligence experts had made a decision to lower the threat level from its highest rating "critical", meaning an attack could be imminent, to "severe".