Britain's Interior Minister Amber Rudd said there is a possibility the perpetrator, Salman Abedi, did not act alone and confirmed he was known to security services.
The suicide bomber who killed 22 people, including children, in Manchester likely did not act alone, Britain's interior minister said on Wednesday as soldiers were being deployed to key sites to help prevent further attacks.
The police triggered Operation Temperer, the security measure allowing military personnel to be deployed at sensitive security points like rail stations, government buildings and other destinations or events with big crowds.
Four men have been arrested in Manchester on Wednesday in an investigation into the suicide bomb attack, Greater Manchester Police said.
TRT World's Sarah Morice is in Manchester bringing the latest.
The official threat level in the country was raised late on Tuesday to its highest level, "critical", meaning an attack is expected imminently.
British-born Salman Abedi, 22, was identified on Tuesday as the young man behind the deadly bomb blast at the end of US pop singer Ariana Grande's concert at Manchester Arena on Monday, attended by thousands of children and teenagers.
"It seems likely, possible, that he wasn't doing this on his own," Interior Minister Amber Rudd told BBC radio.
Rudd also said Abedi had been known to security services before the bombing.
She said up to 3,800 soldiers would be deployed on Britain's streets, freeing up police officers to carry out patrols and investigatory work.
The victims who have been identified as yet include an eight-year-old girl, two teenage girls and a 28-year-old man. A Polish couple who had come to collect their daughters after the concert also died, Poland's foreign minister said. The daughters were safe.
20 in critical care
The bombing also left more than 60 people wounded, some with life-threatening injuries.
"We are now treating 64 individuals ... of those approximately 20 are receiving critical care, that means very urgent care," Jon Rouse, chief officer for health and social care services in the Greater Manchester area, told Sky News.
"There is damage to major organs, major injuries in terms of limbs and some of these individuals are going to need very long term care and support. These are highly traumatic injuries."
The Manchester attack was the deadliest in Britain since July 2005, when four suicide bombers killed 52 people in coordinated attacks on London's transport network.
TRT World's Sarah Morice talks to Furquan Naeem, who went to the same school as Abedi.
What do authorities know so far
US security sources, citing British intelligence officials, said Abedi was born in Manchester in 1994 to parents of Libyan origin.
Rudd told the BBC she believed Abedi had recently returned from Libya.
"Yes, I believe that has been confirmed. When this operation is over, we will want to look at his background and what happened, how he became radicalised and what support he might have been given," she said.
British investigators told French authorities that the attacker had probably travelled to Syria, French Interior Minister Gerard Collomb said on Wednesday.
"Today we only know what British investigators have told us - someone of British nationality, of Libyan origin, who suddenly after a trip to Libya, then probably to Syria, becomes radicalised and decides to carry out this attack," Collomb told BFMTV.
Collomb also said the attacker had links to Daesh.
Pressed on how he knew Abedi had been to Syria, Collomb said this was the information that French and British intelligence services had.
The father of the alleged Manchester attacker, Ramadan Abedi, denies his son is linked to militants or the suicide bombing and said his son is innocent.
He said he spoke to his son, Salman Abedi, five days ago and he was getting ready to visit Saudi Arabia to spend the holy month of Ramadan with family and that he sounded "normal." He also confirmed that his son visited Libya a month-and-a -half ago.
"We don't believe in killing innocents. This is not us," he told AP.
How did Mancunians react?
Thousands of Manchester residents gathered in the centre of the northern English city on Tuesday in a vigil in a show of defiance against the attacker
The vigil drew representatives of different religions who, one by one, condemned Monday evening's bombing, which ripped through a crowd leaving a show by US singer Ariana Grande.