A manhunt is under way to find the person behind Friday’s rush hour Tube bombing in south-west London.
Police said they were « chasing down suspects » and had hundreds of officers trawling through CCTV following the District Line attack, which injured 29.
The UK terror threat has been raised to critical – the highest level – meaning an attack may be imminent.
The Islamic State group has said it was behind the bomb, which was detonated at 08:20 BST at Parsons Green station.
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The station reopened in the early hours of Saturday.
The Metropolitan Police’s Assistant Commissioner Mark Rowley said it was « very routine » for IS to claim the attack « whether or not they’ve had any previous engagement with the individuals involved ».
He asked the public to remain « vigilant », but said people should « not be alarmed ».
BBC home affairs correspondent Danny Shaw said it was notable that the police had use the word « suspects » in their statements, indicating that they may be looking for more than one attacker.
He told BBC Radio 4’s Today programme: « The working assumption at Scotland Yard and MI5 must be that there is not just one person behind this, but at least one, and that there are others that assisted or encouraged the person to plant this device. »
Announcing the change in the UK threat level, Prime Minister Theresa May said the military would be providing support to police and would replace officers on guard duty at national infrastructure sites that are not accessible to the public.
The use of the military to assist police is part of the first phase of Operation Temperer, a government plan to deploy troops to help police following major terrorist attacks, which was activated for the first time on 23 May following the Manchester Arena attack.
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Mrs May said: « The public will see more armed police on the transport network and on our streets, providing extra protection.
« This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses. »
Analysis: A ‘critical’ change
By BBC security correspondent Frank Gardner
This is the fourth time the UK national terror threat level has been raised to « critical » since the system was made public in 2006.
The last time was in May this year following the Manchester Arena bombing, when it was wrongly thought that the bomb-maker was still at large and could strike again.
In the case of Parsons Green it is perhaps surprising that it took the Government so long – over 12 hours – to raise the threat level to critical, under advice from the Joint Terrorism Analysis Centre, when it was obvious the perpetrator had neither died nor been caught.
Each time the level has gone to critical, it has only stayed at this highest level of alertness for three to four days – this is partly as it involves an unsustainably high tempo for the police, intelligence and security services.
Extra patrols are mounted on the streets of London, covert surveillance is stepped up and troops are deployed to free up police officers to focus on the main effort: catching the bomber before he can plant another device.
But the very fact that yesterday’s attack took place with no warning shows this system is only a broad guide to the threat and simply reflects the latest assessment.
Passengers described the bomb, which was in a supermarket carrier bag, as a « fireball ».
Patients were taken to four London hospitals, including one with a specialist burns unit.
It is understood the device had a timer but the BBC’s security correspondent Frank Gardner said the bomb appeared not to have gone off properly.
Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said.
Police urged anyone who took pictures or videos at the scene to upload them to ukpoliceimageappeal.co.uk.
Speaking to Today, Security Minister Ben Wallace said more needed to be done by internet companies to tackle the spread of information online which inspires attacks – including manuals of how to build bombs like the one used in Parsons Green.
« We take a lot down, but sometimes, very quickly, these things reappear, so we go at them again, » he said.
« We are not going to be like some other countries in the world who switch off their internet or just tell companies they can’t operate.
« We have to find a way to build pressure on internet companies to get them to invest some of their vast profits into the technologies to make sure we take down content very quickly or don’t even let it surface. »
What does terror threat level mean?
By Dominic Casciani, home affairs correspondent
A word of caution about « imminence ».
The terror threat level was previously raised to critical in May after Manchester.
Then it was lowered again days later after it became clear to intelligence assessors in the Joint Terrorism Analysis Centre that an attack wasn’t imminent.
Then we had two more incidents – Borough Market/London Bridge and Finsbury Park.
What does this tell us?
Intelligence is usually fragmentary.
Analysts sometimes only have glimpses or impressions of what they think is going on.
It’s an imperfect world.
‘I could see a fireball’
Anna Gorniak, who was in the same Tube carriage as the explosion, said: « I could see a fireball filling the carriage and coming our way. At that moment, I started to run.
« In my mind I was praying, I probably thought for a second, ‘That’s it, my life is over.' »
Peter Crowley was sitting in the carriage, travelling from Wimbledon, when the explosion happened.
He said his head was burned by a « really hot intense fireball above my head » and added: « There were people a lot worse than me. »
Chris Wildish told BBC Radio 5 live he saw a bucket in a supermarket bag with « low-level flames coming out of it » by the door of the rear carriage.
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