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.
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.
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.”
