Intelligence service MI5 will review its processes following the Manchester bombing, as questions continue over how much the authorities knew about the assailant.
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".
Abedi killed himself and 22 others when he detonated a bomb outside a packed Ariana Grande concert at Manchester Arena. Some 119 people were injured.
A source had told the Reuters news agency 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.
Home Secretary 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 the BBC.
Earlier, police made a 16th arrest in connection with the Manchester suicide bombing, the deadliest attack in Britain for 12 years.
A 23-year-old man was arrested on suspicion of terror offences in Shoreham-by-Sea, Sussex.
It comes as surveys suggest Theresa May - who as Home Secretary oversaw the police and domestic intelligence agency - might not win the landslide predicted just a month ago.