LIVE - Updated at 12:21
Police say Remembrance Sunday explosion in a taxi in Liverpool being treated as terrorist incident but motive unclear.
Nick Aldworth, the UK's former counter-terrorism national co-ordinator, is analysing what police and security investigators will be focusing on that moment.
He told BBC News that what he had seen suggested it was either a "a low yield explosive" or "an intentionally incendiary advice or a much greater explosive that has failed to detonate".
He added that lots of these devices are unstable and can cause fires without being detonated.
He added that "despite the intense fire that followed on from that explosion there are likely to be residual items of evidence" for example shrapnel or nuts and bolts. There is also likely to be evidence to the addresses where the suspects were based.
After the Manchester arena bombing, investigators found chemicals used to make the device, which enabled them to determine the size of device and whether there may be any other devices circulating, he said.
He added that people should be especially vigilant in the move to wards Christmas, and that he hoped that this incident had reminded people of the threat of terrorism, which is currently at the "substantial" level in the UK.
More from PA on the arrest of the fourth suspect:
A cordon remains in place outside a terraced house in Sutcliffe Street, Kensington.
A grey Mondeo car parked adjacent is also taped off as uniformed officers continue to guard the front and side alley of the property.
Shortly after 9.10am a young man was put into the back of a police car outside the address.
Here's the response so far from Boris Johnson, reported by the Guardian political correspondent Aubrey Allegretti:
The prime minister praised the "incredible presence of mind and bravery" of a taxi driver injured in the blast at Liverpool Women's, hospital which has been declared a terrorist incident by police.
Speaking during a visit to a vaccine centre in east London on Monday, he told journalists: "This is an ongoing investigation so I can't comment on the details or exactly what type of incident it was, what type of crime it may have been.
"But it does look as though the taxi driver in question did behave with incredible presence of mind and bravery.
"I've got to say this is something that is an ongoing investigation. I think it would be premature to say much more than that."
Assistant Chief Constable Russ Jackson, from counter-terrorism policing north-west, said the arrested men were believed to be "associates" of the taxi passenger, PA Media reports.
He said officers were looking at his associates, his telephone records and purchases he may have made.
He said police had "attributed" the taxi passenger to both the addresses where officers were currently searching but were uncertain which address he lived at.
"We have got significant items in one of the addresses," Jackson said.
Here's the full report on what we know about the situation so far, from three Guardian reporters, Jamie Grierson, Vikram Dodd and Maya Wolfe-Robinson.
The piece will continue to be updated throughout the day.
Related: Liverpool hospital blast declared a terrorist incident
Speaking to BBC News, the Liverpool mayor, Joanne Anderson, said she had driven past the Liverpool women's hospital this morning, where the incident took place yesterday, and saw "plenty of police presence".
She said: "I'm pretty sure they're there to reassure the community, the police are doing everything they can to make sure we're all safe."
She added "most people are able to get about their business in the city", but noted "we must remain calm but vigilant after this incident".
She also praised the quick response from emergency services. "I can't thank our emergency services enough, fire and police on the scene within minutes, arrests made quickly yesterday, people working through the night and everyone's on standby ready to take action."
Asked about the taxi driver, she said: "What an awful experience and jumping out of the taxi he's lucky to have escaped in the way he did. it doesn't bear thinking about what could have happened.
"I think it's really important that we support him in recovering. It's really important this man is able to recover from what must have been a horrific shock yesterday."
The police are expected to give a press conference shortly on their investigation into the blast in the taxi in Liverpool on Sunday.
In the meantime, Kim Johnson, the MP for Liverpool Riverside, is speaking to BBC News.
She said she understood that "counsellors and police will update us on what's happening because it's an ongoing investigation, there hasn't been a declaration it was a terrorist attack."
She added: "The local authorities need to be commended for responding so quickly and professionally to the explosion that took place yesterday
"It was a shocking incident and things could have been a lot worse but the car did receive a lot of fire damage but there's little evidence to suggest there was a lot of bomb damage.
"It was a very concerning and very scary incident, coinciding with the remembrance event today, but there's lots of speculation about what happened and we need to allow the services to conduct their investigations and not get involved with misinformation."
Rachel Hall here leading the blog for today to keep you updated with all developments related to the Liverpool taxi fire as they happen. If I've missed anything, please do drop me a line at rachel.hall@theguardian.com, or you can reach me on Twitter.
A taxi exploded outside Liverpool Women's hospital in the UK on Sunday morning, killing one and injuring another.
Here's what we know so far:
Related: Liverpool hospital taxi explosion: what we know so far
The Liverpool mayor, Joanne Anderson, has praised the taxi driver for minimising the impact of the disaster in an interview on BBC news.
She said: "The taxi driver in his heroic efforts has managed to divert what could have been an absolutely awful disaster.
"We knew the taxi driver had stood out; the taxi driver locked the doors.
"Our thanks go to him."
She also thanked the emergency services for supporting residents following the incidents on Twitter.
As well as the emergency services, a special thanks also to all the council staff who've worked through the night to help our residents affected by the incident @LiverpoolWomens ???? https://t.co/qJGfFdzFxX
Jackson added that emergency services quickly attended the scene and army ordinance disposal had since confirmed the area was safe. "It quickly remained apparent the passenger remained in the vehicle and was deceased," he said, adding: "We believe we know the identity of passenger but cannot confirm at this time."
He said: "Following discussions with ordinance disposal officers, we are able to confirm that this is being treated as the ignition of an explosive device. Our enquiries also indicate that the device was brought into the cab by the passenger.
The assumption at present is the explosive device was built by passenger in taxi, but that the reason it was taken to the women's hospital was unknown, as is reason for sudden explosion.
He said: "We're aware remembrance events were a short while away from hospital when the ignition occurred. We cannot draw connection but it's a line of inquiry we are pursuing. Our inquiries will seek to understand how device was built, motivation for incident and to understand if anyone else was involved in it."
Kennedy said: "I completely understood communities are concerned, I can assure officers are on ground providing high visibility support. Incidents such as this are very rare but I'd ask people to remain calm but also be vigilant and alert.
"There is no specific threat to the area, but I have asked for patrols to be increased right across Merseyside.
"There's been lots of speculation on social media and false reports of further incidents. I'd urge members of public to rely on official information that will be released on our Twitter feed and website.
"Officers from counter terrorism north west are leading the investigation supported by Merseyside police."
Jackson has also said that MI5 is involved.
Kennedy urged people to contact 999 with any concerns.
Police have declared the explosion at Liverpool Women's hospital a terrorist incident in a news conference.
We heard from Merseyside's chief constable, Serena Kennedy, and Russ Jackson from counter-terrorism policing north west.
Jackson said a fourth 20-year-old man has been arrested following the incident yesterday. This follows the arrest of three men aged 18, 26 and 29 who were arrested in Sutcliffe Street, in the central area of Kensington in Liverpool under the Terrorism Act. They will all be interviewed later today by counter-terrorism detectives.
He confirmed that this was now a terrorism incident.
Jackson also confirmed that the taxi driver picked up a passenger at 11am on Sunday, who asked to be taken to Liverpool women's hospital, 10 minutes away. At drop-off, an explosion occurred within the car.
He said: "Remarkably, the taxi driver escaped from the cab. He's being treated for injuries he sustained and is now released from hospital."
Here's some extra detail on the incident from the Guardian's crime correspondent, Vikram Dodd:
Police investigating the explosion and fireball outside a Liverpool hospital believe they have found materials linked to the explosive device used, after a series of dramatic raids and searches.
Investigators believe a suspected improvised explosive device caused the fireball and explosion shortly after the taxi pulled up outside the entrance of the Liverpool women's hospital.
The explosion at around 11am on Sunday led to a series of armed raids and searches, during which materials consistent with the production of an IED were discovered in Liverpool. Some residents were evacuated.
The cab driver is believed to have been in the vehicle at the time of the explosion and runs away after the device is set off.
The UK terrorism threat level remains at substantial, meaning an attack is likely.