MI5 has launched two urgent inquiries into how it missed the danger posed by bomber Salman Abedi, the Guardian newspaper reported. The BBC said MI5 was alerted at least three times to his "extremist views".
A source had told Reuters last week that Abedi was one of "a larger pool of former subjects of interest" whose risk remained subject to review by MI5 and its partners.
Interior minister Amber Rudd declined to comment on exactly what was known about Abedi when asked in several interviews on Monday. But in a highly unusual move for British authorities, she confirmed that MI5 was reviewing its practices.
"I do think it is right that MI5 take a look at their processes to ensure that they work to the best of the possible limits to make sure that we keep people safe," she told BBC television.
Earlier, police made a 16th arrest in connection with the Manchester suicide bombing at a concert by US singer Ariana Grande, the deadliest attack in Britain for 12 years. There are currently 14 people being held in custody.
Britons head to the polls in 10 days' time to elect a new government. Prime Minister Theresa May's Conservatives have seen their poll lead cut in the wake of the attack and after a U-turn over their social care plans for the elderly.
Surveys suggest May - who as a former interior minister oversaw the police and domestic intelligence agency - might not win the landslide predicted just a month ago.
