British intelligence services fear the expert could have more devices - and suicide bombers - in preparation. The attack at Manchester Arena, at the end of a concert by US pop star Ariana Grande, was carried out by Salman Abedi, police said, a 22-year-old British-born man whose parents had fled the Gaddafi regime in Libya.

Abedi's brother Ismail, 23, was arrested in South Manchester on Tuesday and remains in custody. On Wednesday morning, police made three further arrests while executing warrants in South Manchester.

Armed police and troops have also raided a property on Granby Road in central Manchester around lunchtime. The raid was in a five-level high block of flats called Granby House. Police and firefighters and the army kicked the door in to gain access, according to eyewitnesses.

On Tuesday evening, British Prime Minister Theresa May ordered the army onto British streets to protect key sites, including concerts and football matches - and Buckingham Palace - and to free police to hunt a possible active terror cell.

She raised the country's terror alert to the highest level for the first time in a decade.

Britain's official threat level has not been at critical since 2007 - though Operation Temperer troops were briefly deployed at train stations and airports after last year's Brussels terror attack.

Temperer was designed after the Charlie Hebdo attacks in Paris, as an emergency plan to be activated in the case of multiple attacks on British soil. It was kept secret because there were fears it would look like martial law - until it was accidentally leaked to a public website in 2015 and noticed by the Mail on Sunday.

The country's independent Joint Terrorism Analysis Centre concluded, based on the latest information from police and intelligence services, that a further attack remains highly likely and may be imminent, Mrs May said.

"It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack," she added.

Soldiers will be deployed to protect events including the FA Cup final and key locations including Westminster, Downing Street and some embassies.

But as more information emerged about the attacker another possibility was raised: that he was trained in bomb-making at a terrorist camp in Libya.

The suicide bomber who murdered 22 people and injured 59 at Manchester Arena has been named by police as Salman Abedi, a 22-year-old British-born man whose parents had fled the Gaddafi regime in Libya.

He had been studying business management at Salford University in 2014 but dropped out.

Abedi had been known to security services, British Home Secretary Amber Rudd said, though she would not be drawn on the way he came to their attention.

"It seems likely, possible, that he wasn't doing this on his own," she told the BBC on Wednesday.

He was a known associate of a "prolific" Islamic State recruiter, The Times newspaper reported.

IS had claimed responsibility for the attack but Ms Rudd said she did "not have information" about IS ordering the attack.

An imam at the Didsbury mosque that Abedi used to attend told the Daily Telegraph that Abedi had been a "dangerous extremist" who "showed me the face of hate" after the imam made a speech condemning IS.

Neighbours said Abedi had become increasingly devout and withdrawn. One said a few months ago he had seen Abedi chanting loudly in the street.

A school friend told The Times that Abedi "went to Libya three weeks ago and came back, like, days ago".

On Wednesday French Interior minister Gerard Collomb said Abedi had "probably" travelled to Syria and had "proven" links to IS. Mr Collomb told French television he had received this information from British investigators.

"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," Mr Collomb told BFMTV.

Other British media also confirmed with government sources that the bomber had made trips to Libya, where his parents had returned after Gaddafi's overthrow.

Libya is becoming a "jihadist academy", terror expert Hassan Hassan from the Tahrir Institute for Middle East Policy wrote last year.

The country had "become the centre of a rivalry between the Islamic State and al-Qaeda over the leadership of global jihad".

"The intensity of debates among jihadists and Islamists and the existence of long-standing extremists are turning the country's battlefields into an academy producing a new generation of jihadists from across the continent," he wrote.

In January this year the US used bombers and drones to attack IS training camps in Libya, killing militants the Pentagon said had been involved in plotting terror attacks in Europe.

Police forensic experts were examining explosive residues from the scene of Monday's attack, looking to determine the type of device used, which would give valuable clues as to its origin.

Experts said the size and nature of the explosion suggested it had been a sophisticated device.

Colonel Hamish de Bretton-Gordon, former head of chemical and biological weapons for the British Army and NATO, told the Mirror newspaper: "It shows to me a capability you can't just learn on the internet, so I would be very surprised if the attacker worked alone."

He estimated it would have used about a kilogram of explosives.

Former police officer and counter-terrorism expert Chris Phillips said it looked like a planned attack that would have involved a "recce" [reconnaissance] of the site.

"My worry, and I think the police's worry now, is that this person probably wasn't acting alone and there are other people who need to be captured."

Police will examine phone records to determine Abedi's movements and contacts in the lead-up to the attack.

On Wednesday Manchester police welcomed the raising of the threat level, saying they continued to work with national counter-terrorism policing networks and British intelligence services, and predicted "difficult days ahead".