MI5 has reportedly launched two urgent inquiries into how it missed the danger posed by the Manchester bomber, Salman Abedi.
It comes amid claims he was repeatedly reported to the authorities.
One review was started last week, which will aim to quickly identify any glaring errors, while the other will be more in depth, the Guardian has learned.
The home secretary refused to comment on Sunday on whether opportunities were missed to spot the 22-year-old's deadly terrorist intentions.
Abedi, whose parents come from Libya, is thought to have received terror training in the country before returning to the UK just days before killing 22 people.
Home Secretary Amber Rudd said MI5 was right to review the way it had dealt with Abedi in the past.
"I think it is the right first step.
"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," she told Sky News.
"It is right that MI5 take a look to find out what the facts are.
"We shouldn't rush to a make any conclusions at this stage.
"The important thing at this moment, I believe, is to allow the operation to continue and to conclude."
The inquiries come as security officials warned the threat from terrorism is at an “unprecedented scale”.
Counter-terror police made an arrest in West Sussex in connection to the Manchester suicide bombing.
Police have raided multiple addresses - including an address in Chester and another in Manchester in the early hours of Monday.
The number of people now in custody in connection with the attack now stands at 14.
The parents of one of the victims has urged the Government to "open its eyes" to the terror threat facing the UK.
Georgina Callander's family said the 18-year-old's life had been cut short by "evil, evil men prepared to ruin lives and destroy families".