LONDON — An 18-year-old man has been arrested in connection with the attack that injured 30 at a packed London subway station, police said Saturday.
The teen was detained by police near the port of Dover, a key point of entry and exit in the southeast of the country.
Police cautioned that they were still on alert after the Friday rush hour incident, in spite of the "significant" arrest.
A raid was underway Saturday afternoon outside of London in the county of Surrey, where investigators evacuated a residential address and surrounding homes, the Metropolitan Police Service said. A search of the address remained ongoing as a precautionary measure, and there were no immediate reports of other arrests.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical," Neil Basu, the senior national coordinator for counterterrorism policing, said in a statement.
“The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation," he said.
Related: Nannies, Parents Race to Find Kids After London Subway Attack
London Mayor Sadiq Khan responded Saturday to the earlier arrest, saying that "there will still be significant activity today and over the days ahead."
"London will never be intimidated by terrorism," he added. "We will always defeat those who seek to harm us and destroy our way of life."
The detonation at Parsons Green Tube station in southwest London injured 30 people, officials with the nation's health service in London said Saturday. None of their wounds — mostly superficial burns — were considered serious or life-threatening.
Prime Minister Theresa May subsequently announced that the nation's threat level for international terrorism had been increased to critical, its highest level, meaning another terror attack may be imminent.
Britain deployed hundreds of soldiers at strategic sites on Saturday to free up police to hunt those behind the incident.
The arrest comes after multiple law enforcement officials briefed on the investigation said Friday evening that authorities in the U.K. had identified a person they believe is responsible for the attack.
London subway stations are well-equipped with surveillance cameras.
ISIS made its customary claim of responsibility for the attack, without providing any supporting evidence.
Photos of the incident shared on social media showed a bucket on fire in the subway car, with wires apparently protruding from the plastic bag the container had been carried in.
Luke Walmsley, 33, who was six train cars up from the fire described “complete pandemonium, complete terror" aboard the train.
“We heard the first scream and we saw a flash and smoke and people started running immediately,” he said.
Citing two intelligence officials briefed on the incident, NBC News Investigations reported that U.K. authorities were trying to ensure there were no additional devices. The officials added that authorities believe the contents of the 5-gallon bucket detonated prematurely. It did not explode — but instead "flashed" and burned. The device, which was set off by a timer, was left next to one of the subway car's doors.
After reviewing the photo of the burning bucket, NBC News security analyst Duncan Gardham said that it appeared that a homemade detonator possibly involving Christmas lights had "activated but failed to set off the main charge."
He added: "That means the bomb-maker has got a long way down the road to making a viable device."
Parsons Green station reopened on Saturday morning as the city's bustling transit system returned to relative normalcy after a day of disruption.
London’s Tube and bus network was the target of a series of coordinated suicide bombings on July 7, 2005. Often referred to as “7/7,” the attacks during the morning rush hour killed 52 people and seriously wounded hundreds.
The U.K. has been the target of three previous deadly terror attacks this year. A total of 33 people have since been killed in vehicle-and-knife rampages on London Bridge and nearby Borough Market as well as Westminster Bridge. A suicide bomber also targeted an Ariana Grande concert in Manchester.
