LONDON -- The British government lowered the country's official terrorist threat level Sunday after a second man was arrested in connection with the attack on a London subway train where a bomb partially exploded.
The downgrading of the threat level from "critical" to "severe" means authorities no longer believe an attack is imminent. The "severe" classification, the second highest level of alert, is based on the assessment that an attack is "highly likely."
Home Secretary Amber Rudd said the easing of the alert indicates that police and security services are making "good progress" in the sprawling investigation into the attack on a subway train that injured 30 people during the rush hour Friday morning. The Islamic State has said the subway attack was carried out by one of its affiliated units.
Rudd said Saturday that it was "much too early" to say whether those behind the attack had been known to 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."
Rudd cautioned that the investigation was ongoing.
The second arrest came the same day police detained an 18-year-old man "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism" in connection with the bombing.
That teenager was arrested in the southeastern county of Kent, in the departure area of the Port of Dover, where ferries leave for France. A 21-year-old man was arrested Saturday shortly before midnight in the west London borough of Hounslow, the Metropolitan Police force said in a statement.
The force said the second suspect was being held under the Terrorism Act and questioned at a south London police station Sunday, but has been neither charged nor identified.
Police on Sunday also began an urgent search of a property in the southwestern suburb of Stanwell that authorities said was linked to the latest arrest.
"There are now two searches continuing at addresses in Hounslow and Surrey and we are getting a greater understanding of the preparation of the device," Assistant Commissioner Mark Rowley, head of counter terrorism policing, said in a statement.
Police continued searching a home in Sunbury, another southwestern London suburb where neighbors were evacuated on Saturday.
During the attack on a stopped train at the Parsons Green station, a bomb hidden in a plastic bucket inside a supermarket freezer bag only partially exploded, sparing the city much worse carnage.
The two arrests indicate police and security services believe the attack was part of a coordinated plot, not the act of a single person.
"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," Metropolitan Police counter-terrorism coordinator Neil Basu said.
Police have identified 121 witnesses to the attack at the Parsons Green station on Friday and spoken to 100 of them already. Officers continue to trawl through hours of closed-circuit television footage and videos and photographs sent by members of the public, Basu said.
Residents of the Sunbury neighborhood where an armed police search started Saturday were evacuated in a rush and kept away for nearly 10 hours before they were allowed to return to their homes.
The property belongs to an elderly couple who have for years taken in foster children, including refugees from conflict zones in Syria and Iraq.
The pair -- Ronald Jones, 88, and his wife, Penelope Jones, 71 -- have been honored by Queen Elizabeth II for their work with children in need of a stable home.
A friend, Alison Griffiths, said the Joneses are "great pillars of the community" who have taken in several hundred children in the last 40 years.
Neighbors said the couple were staying with friends during the search.
Neighbors also said two young men had been staying with them recently. Police have not provided details about the extensive search, which began several hours after an 18-year-old suspect in the subway bombing was arrested at Dover's ferry port.
Police said they were "working to support displaced residents and to get them back into their homes as soon as possible."
Rowley said that as the workweek began, specialized teams of investigators would maintain a "strong visible presence throughout the capital and across the U.K.," and that the public would "still see that high level of policing presence, some armed, some unarmed."
The improvised explosive device placed on the subway train only partially detonated, limiting the number of injuries.
The National Health Service says all but one of the 30 people treated for injuries has been released from the hospital. One person is still being treated at Chelsea and Westminster Hospital, which has a special unit for treating burns.
Officials have raised the number of injured from 29 to 30. That includes 19 people who were taken from the explosion site at Parsons Green station to the hospital and 11 who came in for treatment later on their own.
Home Secretary Rudd said the casualties would have been far higher if the bomb had fully detonated. Frustrated by the string of terrorist attacks in recent months, she said officials will have to work harder to make bomb components more difficult to obtain.
The attack is the fifth this year in the U.K. and Londoners are growing accustomed to the sight of armed police patrolling the transport network. Police said Thursday that terrorism-related arrests had risen 68 percent over the past year.
Earlier this year, assailants with vans and knives attacked passers by on Westminster Bridge and London Bridge in two separate strikes, and a van was driven into worshippers outside a mosque in Finsbury Park. A suicide bomber attacked a concert venue in Manchester in May, killing more than 20 people including children and mothers. The terror threat level was raised to critical after that attack and lowered four days later to severe, meaning an attack is considered highly likely but not imminent.
Most of the attacks have been claimed or praised by the jihadist group Islamic State.
Information for this article was contributed by Gregory Katz of The Associated Press; Emma Ross-Thomas, Thomas Penny and Benjamin Katz of Bloomberg News; and Ceylan Yeginsu of The New York Times.
A Section on 09/18/2017