An 18-year-old man is being questioned by counterterrorism officers after being arrested in Dover in the wake of the Parsons Green bomb attack in west London.
Police described the arrest as “significant” after seizing the man in the port area of Dover at 7.50am before taking him from Kent to a south London police station.
It was the first breakthrough in the huge investigation to track down the individual or network behind the attack on a London tube train.
Six hours later the inquiry widened further when armed police raided a home in the suburb of Sunbury-on-Thames, Surrey, in an operation during which residents were ordered to evacuate surrounding homes immediately.
Mojgan Jamali, a mother of three who lives in Cavendish Road, said: “They told me to leave. They said: ‘You have one minute to get out of the house and get away’.”
The property that was raided is registered to foster carers Penelope Jones, 71, and her husband Ronald, 88, who were awarded MBEs in the 2009 New Year honours list for services to children and families.
With the terror threat level in the UK still at its highest, hundreds of officers are attempting to piece together how many individuals were behind the planting of the improvised explosive device on a packed rush-hour tube carriage on Friday morning. At least 30 people were injured in the attack, and three remain in hospital.
The possibility of a cell remaining at large was raised by the Met’s deputy assistant commissioner, Neil Basu, who said police were “keeping an open mind” on whether there was more than one person involved in the attack and its planning.
“If there are other people responsible it’s our job to find them,” he added.
Earlier Scotland Yard’s most senior officer, Cressida Dick, speaking as she joined officers on patrol in central London, declared the capital would not be cowed by terrorism and urged people to remain vigilant in the face of what she described as a “shift in threat”.
Friday’s terror attack was the fifth in the UK this year with at least seven other significant plots known to have been foiled, making it the most sustained period of terror activity in England since the IRA bombing campaign of the 1970s.
Armed police are to remain on duty throughout the country while troops have been deployed to guard key infrastructure and transport hubs following the Joint Terrorism Assessment Centre’s decision on Friday to raise the threat level to critical, indicating another attack is expected imminently.
Although police were keen to play up the significance of the arrest on the south coast, the Met said it would not be revealing where the suspect was from, whether he was the suspected bomber or the precise details of the offence for which he was arrested.
The location of the arrest has, however, led investigators to believe the 18-year-old was intending to head abroad. Dover is the busiest ferry hub in Europe with dozens of crossings a day to Calais alone. A police presence remained around the port area with one witness describing seeing three unmarked police cars with their sirens on leaving the Eastern Docks ferry terminal at around 12.30pm.
The home secretary, Amber Rudd, confirmed that the arrest was “very significant” but also that the attack could easily have caused significantly more harm, with the improvised device seemingly failing to detonate fully.
Speaking after an emergency meeting of the government’s emergency Cobra committee, Rudd said: “There is no doubt that this was a serious IED, it was good fortune that it did so little damage.”
The home secretary then appeared to indicate that the suspected attacker was a man as she pledged to make it harder for individuals to construct similar bombs in the future.
“We will have to make sure to take all steps that we can to ensure the sort of materials that this man was able to collect become more and more difficult to combine together,” she said.
Scientists from the government’s forensic explosives laboratories in Kent are continuing to inspect the bomb which is believed to have used TATP, an explosive compound made from hydrogen peroxide, commonly sold as hair bleach or other household products.
Another central strand of the investigation is focusing on CCTV coverage with large numbers of specialist officers combing through footage to establish when the device was placed on the train and the bomber’s movements before and afterwards.
The security minister Ben Wallace suggested that CCTV images of the bomber could be released as part of the hunt for those responsible, but Scotland Yard subsequently denied that was the plan. By lunchtime on Saturday, the Met said 100 witnesses had been interviewed and 180 images and videos sent to detectives by members of the public, a number that is expected to increase as the investigation evolves.
“The public’s help remains crucial, and I urge anyone with information, no matter how significant they think it is, to continue letting us know,” said Basu.
Islamic State has claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.
It also remains unclear whether the bomber was already known to the British security services, with Rudd stating it was “much too early to say” whether the bomber was part of the current intelligence picture. On Friday the US president Donald Trump sent tweets suggesting British intelligence officials were aware of who was behind the attack.
Another unknown is how long Operation Temperer, the use of military personnel to free up officers for patrols, will remain in force. The plan is believed to allow up to 5,000 troops to be deployed in support of the police.
