The United Kingdom lowered its terror threat level on Sunday from "critical" to "severe," just hours after police arrested a second suspect in connection with the London subway attack.
Home Secretary Amber Rudd, however, cautioned that the investigation is ongoing and Britain still faces a substantial threat.
"Severe still means that an attack is highly likely, so I would urge everybody to be vigilant but not alarmed," she said.
Mark Rowley, who heads the police counter-terrorism operation, added that London commuters will likely still see an increased police presence during the next few days.
A second man has been arrested in connection with the London subway attack.
The 21-year-old was arrested late Saturday in west London under the Terrorism Act. So far, the unidentified man has not been charged.
Earlier on Saturday police arrested an 18-year-old in the departure area of the Port of Dover, which is where ferries leave for France.
Counter-terrorism police investigating the London bombing also searched the property of an elderly couple who have been honored by Queen Elizabeth II for taking in foster children in need of a stable home.
Ronald Jones, 88, and his wife Penelope Jones, 71, are reportedly staying with friends while police conduct their search. Police have not yet provided any details about the extensive search.
Authorities said in a statement that they've already interviewed 100 witnesses and are still reviewing CCTV footage of the incident.
Police arrested an 18-year-old man Saturday morning in connection to an explosion on a train at a London Underground station Friday morning.
Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism policing, called the arrest "significant" in a statement, according to CNN.
"This arrest will lead to more activity from our officers. For strong investigative reasons, we will not give any more details on the man we arrested at this stage," he said.
ISIS has claimed involvement in the incident, hours after the attack.
The suspect is being held at a local police station and will be transferred to a south London police station, officials said.
At least 30 people were injured in the attack. Three of the 30 people remained in a London hospital on Saturday.
London police say they are making "good progress" in their investigation of the subway bombing.
The incident occurred Friday morning on a subway train at the Parsons Green station in southwest London.
Assistant Commissioner Mark Rowley said that law enforcement will be on the streets on London Friday night.
Rowley said, “We have remnants of that device. We are chasing down suspects.”
Authorities are aware of one device potentially used in the subway bombing. However, the investigation is still underway.
British police said a tweet by President Trump in which he suggested the "loser terrorist" responsible for the London subway attack was "in the sights of Scotland Yard" was "pure speculation" and "unhelpful."
Another attack in London by a loser terrorist.These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!— Donald J. Trump (@realDonaldTrump) September 15, 2017
Loser terrorists must be dealt with in a much tougher manner.The internet is their main recruitment tool which we must cut off & use better!— Donald J. Trump (@realDonaldTrump) September 15, 2017
Trump's tweet was "pure speculation given we don’t know who is involved. Any speculation is unhelpful," a police spokesman told CNN.
The president subsequently tweeted that the "travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!"
The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!— Donald J. Trump (@realDonaldTrump) September 15, 2017
A manhunt is underway for the perpetrator or perpetrators of the attack as health officials say the number of people injured has risen to 22. None of the injuries are believed to be life-threatening.
Here's an aerial view of the scene.
London’s Metropolitan Police Service (MPS) on Friday declared a fire on the city’s subway as a “terrorist incident.”
“[It’s] too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met’s Counter Terrorism Command,” the MPS said.
The MPS said counterterrorism officers are now probing early Friday’s incident at the London subway’s Parsons Green station, which resulted in a “number” of people getting injured.
Passengers reported an explosion in a train carriage shortly after 8 a.m. local time during the morning’s rush hour, and several people appeared to have burn injuries following the incident.
Some Twitter users on Friday voiced outrage at the reported explosion in London, which is the latest in a series of extremist attacks in the city this year.
The London Ambulance Service said that 18 people had been taken to hospitals after the fire at the Parsons Green station.
Commuters reported seeing people with burns on their faces and bodies following the incident, and the service noted one person’s injuries were believed to be serious or life-threatening.
Friday’s explosion resulted in panic, with passengers stampeding in fear after the explosion in the train’s carriage.
Photos taken inside the District Line train showed a white plastic bucket inside a supermarket shopping bag, with flames shooting out and wires seemingly visible.
London Mayor Sadiq Khan said that the city “will never be intimidated or defeated by terrorism.”
“[London] utterly condemns the the hideous individuals who attempt to use terror to harm us and destroy our way of life,” he said.
Khan added he would join a meeting of the British government’s emergency committee chaired by Prime Minister Theresa May later that afternoon.
“My thoughts are with those injured at Parsons Green and [the] emergency services who are responding bravely to this terrorist incident,” May tweeted.
The Associated Press contributed to this report.