Update: Having mobilized hundreds of soldiers to protect key landmarks across London, the British police launced a huge manhunt for “a network” of accomplices who may have helped Salman Abedi build the suicide bomb and "who could be ready to kill again." Part of yesterday's hike in the UK risk threat assessment is the fear that Abedi could have been working as part of a group of accomplices with possible links to militant groups who have the competence to plot and execute suicide bombings.

"The question is: Was he acting alone or was he part of a network of others who want to kill. That is what the investigation is focusing on," a source told Reuters adding that "the concern is that there may be others out there who helped him to make the bomb. Making a bomb of this sort requires a certain level of expertise and competence," the source said.

At an afternoon press conference, Manchester Chief Constable Ian Hopkins said it is clear “this is a network we are investigating.” He declined to give any further details on the investigation. British police have declined to provide many details about the suspect, but a U.S. official has said he was a British citizen of Libyan descent. British officials believe he had recently returned from Libya. Hopkins also wouldn’t comment on whether police had found anyone who made the explosive device used in the attack.

Moments ago Reuters reported that the younger brothers of the bombing suspect was arrested in Libya on suspicion of ISIS ties:

HASHEM ABEDI, YOUNGER BROTHER OF MANCHESTER ATTACKER, ARRESTED IN TRIPOLI BY COUNTER-TERRORISM FORCE ON SUSPICION OF ISLAMIC STATE LINKS

BROTHER OF MANCHESTER ATTACKER ARRESTED IN TRIPOLI WAS PLANNING "TERRORIST ACT" IN THE LIBYAN CAPITAL - COUNTER-TERRORISM FORCE

Hashem Abedi confessed to being in the U.K. during preparations for Monday’s attack and was aware of all plans, said Ahmed Dagdoug of Radaa, one of several large militias responsible for security in Tripoli quoted by the WSJ. Radaa, which holds sway over part of the Libyan capital, said the younger Abedi was arrested late Tuesday in the city as he picked up a wire transfer of 4,500 Libyan dinar, or about $3,260, sent by his late brother, Salman.

The group’s spokesman, Mr. Dagdoug, said it was also holding Abedi’s father, Ramadan Abedi, to aid in the investigation of the attack, which killed 22 people outside a concert by American singer Ariane Grande. It wasn’t immediately clear whether the Libyan group was in contact with British investigators, who on Tuesday in Manchester arrested a man one Western official identified as 23-year-old Ismail Abedi, another brother of the suspect.

Meanwhile, in the UK the police arrested at least three more men as part of their investigation into the suicide bombing as authorities said they are pursuing a “network” in connection with the attack. The arrests in Manchester take the total currently in custody to four. Under British law, a person can be taken into custody in a terrorism investigation and held up to 14 days without charges.

As reported earlier (see below), U.K. investigators told French authorities that Abedi had probably also traveled to Syria, according to the French interior minister.

A leading theory is that the attacker may have received specialist training abroad or that there is a technician in the U.K. who constructed the bomb, the Western official said. “To cause this many fatalities it has to be a viable device of a certain level of sophistication,” the official said, adding that it didn’t seem like something Abedi could have done by himself. Officials were still in the initial phase of the investigation, the person said.

Earlier, Home Secretary Amber Rudd earlier told the British Broadcasting Corp. Abedi that Abedi was previously known to security services “up to a point.” “When this operation is over, we will want to look at his background and what happened, how he became radicalized and what support he might have been given,” she said.

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Earlier:

Just hours after the UK raised its terror alert to Critical, or the highest possible, for the first time in ten years, Britain’s Interior minister Amber Rudd said that Salman Abedi, the Manchester suicide bomber who killed 22 people at a concert venue, and had recently returned from Libya had likely not acted alone and troops were being deployed to key sites across Britain to help prevent further attacks according to the FT.



Salman Abedi, the suspect in the Manchester attack

Rudd said on BBC radio that the bombing was “more sophisticated than some of the attacks we’ve seen before, and it seems likely, possible, that he wasn’t doing this on his own.” She said Abedi had been known to security services before the bombing. Asked about reports that Abedi had recently returned from Libya, Rudd said she believed that had now been confirmed.

Rudd said up to 3,800 soldiers could be deployed on Britain's streets, taking on guard duties at places like Buckingham Palace and Downing Street to free up police to focus on patrols and investigation. An initial deployment of 984 had been ordered, initially in London, then elsewhere. The minister also scolded U.S. officials for leaking details about the investigation into the Manchester attack before British authorities were prepared to go public.

Separately Rudd's French counterpart said Abedi had links with Islamic State and had probably visited Syria too. According to Reuters, French Interior Minister Gerard Collomb said British investigators had told French authorities Abedi had probably travelled to Syria as well.

"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 radicalized and decides to carry out this attack," Collomb told BFMTV. Asked if he believed Abedi had the support of a network, Collomb said: "That is not known yet, but perhaps. In any case, (he had) links with Daesh (Islamic State) that are proven."

The Islamic State promptly claimed responsibility for the Manchester attack, but there were contradictions in its accounts of the action and a lack of crucial detail.

As Collomb was speaking in France, Rudd was asked by the BBC about the fact that information about Abedi, including his name, had come out from the United States and whether she would look again at how information was shared with other countries. "Yes, quite frankly. I mean the British police have been very clear that they want to control the flow of information in order to protect operational integrity, the element of surprise, so it is irritating if it gets released from other sources and I have been very clear with our friends that should not happen again."

Asked whether the U.S. leaks had compromised the investigation, she said: "I wouldn't go that far but I can say that they are perfectly clear about the situation and that it shouldn't happen again."

According to Bloomberg, "it is rare for the U.K. government to publicly criticize the U.S. and in such blunt terms. The rebuke raises the risk that key allies could become more reluctant to share vital security information with the world’s superpower."

The bomber’s name, Salman Ramadan Abedi, was first revealed early on Tuesday by CBS in the U.S. and hours later the U.K. authorities put out a statement refusing to confirm the information until a formal identification had been completed. The police said any speculation would be “unhelpful and potentially damaging” to the investigation. It was only much later in the day, that the U.K. confirmed his identity.

Separately, Manchester Police said on Wednesday morning three men had been arrested “after police executed warrants in south Manchester” in connection with the continuing investigation.

On Tuesday police raided the Abedi family home in the Fallowfield district of south Manchester, in one of three operations carried out as authorities tried to establish whether Abedi was working alone or as part of a network. Family friends and neighbours said Abedi’s parents were originally from Libya and recently returned to the country.

The son of Libyan immigrants, British-born Abedi, 22, blew himself up on Monday night at the Manchester Arena indoor venue at the end of a concert by U.S. pop singer Ariana Grande. His 22 victims included an eight-year-old girl, several teenage girls, a 28-year-old man and a Polish couple who had come to collect their daughters. The bombing also left 64 people wounded, of whom 20 were still receiving critical care for highly traumatic injuries.