The Islamic State claimed responsibility for the bombing attack at the Parsons Green station of the London Underground, in a quiet, affluent part of West London. A crude explosive, wrapped in a plastic bag that was concealed in a bucket, detonated during the morning rush hour in a train at the station.
Witnesses said the carriage was engulfed in flames, and dozens of passengers were trampled as they tried to escape. No one was killed, but at least 30 people suffered injuries, including burns and fractures. Several people were hospitalized.
After the bombing, Britain raised its terrorism threat level to “critical,” the highest level, which means another attack is “expected imminently.”
Britain was hit by a terrorist attack on Friday morning, when a crude device exploded on a crowded London Underground train. Passengers described seeing a wall of fire.
Home Secretary Amber Rudd announced on Sunday that the threat level had been downgraded to “severe,” meaning that an attack was highly likely. She urged the public to be vigilant.
Ms. Rudd said on Saturday that it was “much too early” to say whether those behind the attack had been known to the authorities. On Sunday, she also cast doubt on the Islamic State’s claim.
“It is inevitable that so-called Islamic State, or Daesh, will reach in and claim responsibility,” she told the BBC, using an Arabic acronym for the group. “We have no evidence to suggest that yet.”
The Metropolitan Police said, “At this stage we are keeping an open mind around whether more than one person is responsible for the attack, and we are still pursuing numerous lines of inquiry.”
The British Transport Police Firearms said on Twitter that it had placed extra officers on duty to patrol stations.
On Sunday about 12:30 p.m. local time, police forensics teams put up cordons in a West London neighborhood and began searching a property there. Residents were not allowed to leave, according to a local television report, and the “sizable” operation was connected to the arrest of the 21-year-old.
The police confirmed they were also searching a house in Stanwell, a village in the county of Surrey.
On Saturday, officers began searching another house, this time in Sunbury-on-Thames, after evacuating surrounding buildings and setting up cordons. Some residents were offered transportation to a local rugby club, and others were allowed to go to the homes of relatives nearby.
The house being searched was occupied by an older couple known for fostering hundreds of refugee children, according to local news reports. The couple were appointed Members of the Order of the British Empire for services to children and families, and Queen Elizabeth II presented them with the honor at Buckingham Palace in 2009.
Neighbors said the couple were staying with friends during the search.
The police said they were “working to support displaced residents and to get them back into their homes as soon as possible.”