iStock/Thinkstoc(LONDON) — An 18-year-old man was arrested Saturday morning in Dover, England, in connection with Friday’s terror attack on a London Underground train, for which ISIS has claimed responsibility, police said.

Kent Police arrested the man in Dover’s port area, about 75 miles from London, at approximately 7:50 a.m. local time under section 41 of the U.K.’s Terrorism Act, according to London’s Metropolitan Police Service. The section provides authorities the power to arrest a person suspected of terrorism-related offenses without a warrant.

The suspect remained in custody at a local police station Saturday morning, but he will soon be transferred to a south London police station, police said. 

UPDATES: WHAT WE KNOW NOW

• An 18-year-old man has been arrested in connection with Friday’s apparent bucket bomb explosion on a London Underground train.
• Following the arrest, police searched a residence in Sunbury-on-Thames.
• ISIS has claimed responsibility for the terror attack on the London tube.
• The terror attack is the sixth in Britain this year.
• The blast sent 29 people to the hospital.
• Witnesses described a ball of flames and a loud explosion on the subway car.
• Police said the improvised explosive device did not fully explode.
• The U.K. threat level has been raised to “critical,” meaning an attack is expected imminently.

“We have made a significant arrest in our investigation this morning,” Metropolitan Police Deputy Assistant Commissioner Neil Basu said in a statement on Saturday. “Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.”

“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,” Basu added.

Following the arrest, police on Saturday afternoon evacuated and searched a residential address in Sunbury-on-Thames, a town in Surrey some 15 miles southwest of central London, in connection with the investigation. Neighbors immediately surrounding the address were also evacuated as a precautionary measure, according to the Metropolitan Police Service.

A search of the home was ongoing later on Saturday, and cordons were to remain in place until the operation was complete.

London Mayor Sadiq Khan hailed Saturday morning’s arrest.

“This morning, police have made a significant arrest as part of the investigation into the terrorist attack at Parsons Green station yesterday morning,” Khan said in a statement. “The police investigation is ongoing, and there will still be significant activity today and over the days ahead. I am sure I speak for London when I say we are incredibly grateful to the police and intelligence services for doing everything possible to keep Londoners safe.”

“London will never be intimidated by terrorism. We will always defeat those who seek to harm us and destroy our way of life,” the mayor added.

ISIS claimed responsibility through its Amag News Agency for Friday’s attack, in which an apparent bucket bomb exploded on a London Underground train during the morning rush hour on Friday. The blast propelled a fireball through the passenger car, sending at least 29 people to area hospitals, officials said.

Authorities immediately launched a manhunt for the perpetrators.

“This is a live investigation,” Metropolitan Police Assistant Commissioner Mark Rowley, head of national counterterrorism policing, said Friday morning outside Scotland Yard, the police force’s headquarters. “There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses.”

Following an emergency Cabinet meeting, British Prime Minister Theresa May on Friday said the nation’s threat level was raised to “critical,” the highest level. The public should remain vigilant, she said.

A British government official said Friday’s attack appears to be an isolated incident.

Eyewitnesses told ABC News that the blast happened as the train pulled into the Parsons Green station in Fulham, an affluent, mainly residential area of West London.

“I heard a loud bang, and as I looked to my right. There was a flame; a fireball came through the carriage. … As the doors opened, people then began leaving the train straight away,” said Martin Adams, a security manager for The Walt Disney Company who was riding in the subway car at the time. “I saw some flames coming from what I thought was a blue bag.”

Another person who was at the station said she saw a number of people with what appeared to be facial burns and singed hair after the explosion.

“I saw a couple people with burns. One lady had her hair badly singed by the fire,” said Sally Faulding, who witnessed the panic on the subway platform. “I also saw people injured obviously from having been stampeded on the platform because we were all running. People were falling over.”

The London Ambulance Service said it transported 19 patients to area hospitals and 10 others took themselves, but none of the injuries were serious or life-threatening.

Adams said the situation was managed well by train operators and emergency crews, and passengers remained relatively calm.

“There were no casualties at all on the train. Everybody appeared to get off,” Adams told ABC News. “There was nobody laying on the platform floor. I assessed that there were no serious casualties at that time.”

Police said the improvised explosive device on the train did not fully explode. Apparent images of the device show wires hanging out of a white bucket.

The area surrounding the Parsons Green tube station was evacuated Friday as emergency services continued to work at the scene and officers secured the remnants of the device, police said.

Basu, the senior national coordinator for counterterrorism policing for the Metropolitan Police Service, declared the event as a “terrorist incident.”

Transit officials announced early Saturday morning that the station had fully reopened and that there is “good service operating on the line following yesterday’s incident.”

The Met’s Counter Terrorism Command are investigating after the incident at #ParsonsGreen tube station is declared a terrorist incident

— Metropolitan Police (@metpoliceuk) September 15, 2017

London’s mayor released a statement following Friday’s attack condemning those responsible.

“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” Khan said. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”

U.S. President Donald Trump, who has clashed with Khan in the past, shared his thoughts on Twitter just before 7 a.m. ET.

Trump referred to the suspects being “in the sights of Scotland Yard,” though London officials have not publicly provided any confirmation of that.

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

Law enforcement agencies across the United States said they are closely monitoring Friday’s incident in London.

New York City Police Commissioner James O’Neill said Friday that there were no known threats to the city’s subway system, but reminded passengers to remain vigilant and aware of their surroundings.

However, New York Gov. Andrew Cuomo released a statement Friday saying he was increasing security in public transportation areas across the Empire State.

“Out of an abundance of caution, I am directing state law enforcement to increase security at vital assets across New York, including airports, bridges, tunnels and mass transit systems,” Cuomo said. “The safety and security of New Yorkers is our No. 1 priority, and we remain in close contact with local and federal officials. We remain vigilant, and we stand with the people of London.”

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