MANCHESTER, United Kingdom — Britain ordered soldiers to key sites today and raised the terror alert to maximum after the Manchester concert suicide bombing by a local man of Libyan origin who may have been radicalised in Syria.

Security services believe the suspected bomber, Salman Abedi, was likely to have had help from others in staging the massacre that killed 22 people including one girl aged just eight.

Interior minister Amber Rudd said the 22-year-old had been on the radar of the intelligence community before the attack late Monday at a concert by US pop star Ariana Grande.

Investigators were trying to piece together the last movements of Abedi, a Manchester-born man of Libyan descent whose parents had reportedly fled the former regime of Moamer Kadhafi.

After arresting a 23-year-old man on Tuesday, police said they had arrested three more men today in south Manchester, where Abedi lived.

Abedi was reported to be a former business student who dropped out of university and turned to radical Islam.

French Interior Minister Gerard Collomb said he had "likely" been to Syria, citing information provided by British intelligence services to their counterparts in Paris.

He told French television the suspect "grew up in Britain and then suddenly, after a trip to Libya and then likely to Syria, became radicalised and decided to carry out this attack." "In any case, the links with Daesh are proven," he said, using a term for the Islamic State group, which claimed responsibility for the attack on Tuesday.

Police on Tuesday staged an armed raid on a Manchester address believed to be where Abedi lived, carrying out a controlled explosion to gain entry.

Rudd declined to give any further information about Abedi but told BBC radio "It 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." The minister said she was "not surprised at all" that Islamic State jihadists had claimed the attack but said there was no information yet to confirm the extremist organisation’s active direction.

Prime Minister Theresa May placed the country on its highest level of terror alert – "critical" – for the first time since June 2007, when it was sparked by an attack on Glasgow airport.

Troops will fan out at sites such as Buckingham Palace, Westminster and foreign embassies in London to free up armed police for anti-terror duties.

May said a new attack "may be imminent" and stressed that the soldiers would remain under police command.

The Changing of the Guard, a military ceremony in front of Buckingham Palace popular with tourists, was cancelled on Wednesday and the Houses of Parliament suspended all public events.

’Sing with the angels’ 

The attack was the deadliest in Britain since July 7, 2005 when four suicide bombers inspired by Al-Qaeda attacked London’s transport system during rush hour, killing 52 people.

A Polish couple living in Britain were confirmed among the Manchester victims, along with 15-year-old Olivia Campbell, whose mother had issued heartrending appeals for help when her daughter was still missing.

Next to a photograph of Olivia, Charlotte Campbell wrote on Facebook: "RIP my darling precious gorgeous girl Olivia Campbell taken far far too soon. Go sing with the angels and keep smiling, mummy loves you so much." A total of 59 people were taken to hospital, many with life-threatening conditions. Twelve of them were aged under 16.

Twenty people are still in critical care, officials said on Wednesday.

Britain’s best-selling newspaper The Sun carried pictures on Wednesday of eight-year-old Saffie Rose Roussos, who was killed in the attack, and of Abedi under the words "PURE" and "EVIL" respectively.

 Black armbands 

The plan for the troop deployment, which has never before been used and is codenamed Operation Temperer, was first revealed after the November 2015 Paris terror attacks and is believed to allow up to 5,000 troops to be deployed.

The last time troops were deployed on British streets was after a suspected airliner plot in 2003.

France has already grown used to the sight of rifle-toting soldiers in its major cities under a state of emergency imposed after the attacks on the Bataclan concert hall and elsewhere in Paris.

Earlier on Wednesday, French President Emmanuel Macron said he would seek an extension to the state of emergency until November.

British police had already announced extra security measures for upcoming sporting fixtures including Saturday’s FA Cup football final.

Manchester was the latest in a series of deadly incidents across Europe claimed by IS jihadists that have coincided with an offensive on the group’s redoubts in Syria and Iraq carried out by US, British and other Western forces.

In a city famed globally for its football teams and pop bands like Oasis, The Stone Roses, The Smiths and Take That, showbusiness stars and teams have joined political leaders worldwide in expressing their horror at the carnage.

"Words don’t really do justice for how we all feel. We’re numb," Manchester United’s executive vice-chairman Ed Woodward said, ahead of Wednesday evening’s Europa League final against Amsterdam giants Ajax.

The players will wear black armbands for the game in Stockholm and a minute’s silence will be held. — AFP