A desperate search is on for the man who built the nail bomb that killed 22 people at an Ariana Grande concert in Manchester on Monday night, as fears grow that a second attack is imminent.
Suicide bomber Salman Abedi was a "mule" used to carry out the attack and the bomb maker is still at large, a police source told the Manchester Evening News.
"They don't waste bomb makers," he said.
"The reason we have gone to critical is because he is still out there and the fear is that he will strike again before they get caught."
British Prime Minister Theresa May has raised the country's terror threat level from "severe" to "critical", meaning authorities expect another attack is imminent.
Greater Manchester Police have declined to comment on the search for the bombmaker officially, but confirmed that Mr Abedi did not act alone in carrying out his deadly attack on concertgoers after the US pop stars performance in the city in England's northeast.
Chief Constable Ian Hopkins said the level of police activity in the investigation was "intense and is continuing at a fast pace".
"I think it's very clear that this is a network that we are investigating and, as I said, it continued at a pace, there's extensive investigations going on and activity taking place across Greater Manchester as we speak," he told reporters on Wednesday afternoon.
Suicide bomber's suspicious trip
The Manchester bomber returned from a mysterious trip just days before he detonated the bomb inside Manchester Arena.
It has been revealed that Salman Abedi took a three-week trip to Libya, where his parents hail from, and then carried out the terrorist attack just days after he returned to Manchester.
Police and intelligence officers are now investigating why the suicide bomber took the trip and who he met there.
There are also reports in the British media that Abedi recently visited Syria, a hot bed for Islamic State.
The new details come after police confirmed that Abedi, 22, was not a lone-wolf terrorist but was, in fact, part of a "network".
Abedi died in Monday night's attack, but police have since arrested five people in connection with the atrocity. Among them is Abedi's brother, Ismail, 23.
The fifth man arrested on Wednesday afternoon was found in the Manchester suburb of Wigan carrying with a suspicious package.
In addition to these five UK arrests, another of Abedi's brothers, Hashem, has also been arrested in the Libyan capital of Tripoli on suspicion of having links to Islamic State.
"We have evidence that he is involved in Daesh (Islamic State) with his brother. We have been following him for more than one month and a half," a spokesman for a local counter-terrorism force told Reuters.
"He was in contact with his brother and he knew about the attack."
Earlier in the day back in the UK, heavily armed police swooped on an apartment block in Manchester's city centre as part of their efforts to establish who may have helped Mr Abedi carry out the horrific bombing.
Police blocked off Granby Row and temporarily shut down a railway line as dozens of officers in body armour carrying machine guns raided the apartment block.
Witnesses told CNN they heard a loud bang as the police descended, which was a controlled explosion that allowed officers to gain access to the property.
Residents of the apartment block were evacuated, the Manchester Evening News reported.
Neighbours told the paper that it was believed the flat was being rented out to people via the website Airbnb.
IS has claimed responsibility for Monday night's attack.
British Home Secretary Amber Rudd said Abedi was known to British intelligence services and police "up to a point".
But France's Interior Minister Gerard Collomb went further, telling French TV that intelligence operatives believed he had travelled to Syria and had "proven" links with IS.
"All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack," he told BFM television.
Father says son is innocent
Meanwhile, the father of the Manchester suicide bomber has defended his son, insisting he is innocent of carrying out the attack.
"We don't believe in killing innocents. This is not us," Ramadan Abedi told the Associated Press from Tripoli, Libya.
"We aren't the ones who blow up ourselves among innocents. We go to mosques. We recite Quran, but not that.
He said his son sounded "normal" when they spoke five days ago and said his son was planning a visit to Saudi Arabia.
"There was nothing worrying at all until two days ago (when) I heard the news that they suspect he was the bomber," he said.
Abedi senior denied that his son had ever been to Syria.
Police officer among dead
Fourteen people have now been named among the 22 people who died in the attack, the youngest of whom was eight-year-old Saffie Rose Roussos.
Police have also confirmed that an off-duty female police officer was also killed in the bombing, but she has not been named.
"Very sadly I can confirm one of the victims was a serving officer," Chief Constable Ian Hopkins of Greater Manchester Police said.
"In respecting the families' wishes, I will make no further comment at this stage."
The BBC reported that the officer was a mother of two.
Her husband and two children are all believed to have been injured in the attack.
Manchester health authorities say 64 people remain in hospital, with 20 of those in a critical condition.
"I would also like to confirm that we have spoken to all of the families of those that lay injured in our hospitals and of course we are doing all that we can to support all of them too," Chief Constable Hopkins said.