LONDON (CBSNewYork/CBS News/AP) — British police made a “significant” arrest in the urgent manhunt for suspects a day after the London subway blast that injured more than two dozen people, authorities said Saturday.

Police said that an 18-year-old man was arrested by Kent police in the port of Dover on the English Channel. He is being questioned under the Terrorism Act. Dover is a major ferry port for travel between Britain and France.

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READ: 29 People Hurt In Bombing On Subway Train In London; Terror Threat Level Raised To Critical

“We have made a significant arrest in our investigation this morning,” Deputy Assistant Police Commissioner Neil Basu said. But he warned that the investigation was ongoing and the terrorist threat level remains at “critical,” meaning a government task force that includes the security services believes another attack is imminent.

Basu’s comments suggested that other dangerous suspects may still be at large.

“At this stage, we’re keeping an open mind around whether more than one person is responsible for the attack, and we are still pursuing numerous lines of inquiry and at a great pace,” he said.

The 18-year-old suspect hasn’t been charged or identified. Police say he will be brought to a south London police station for more questioning. Police haven’t said if he is suspected of planting the bomb or if he played a supporting role in a possible plot.

After the arrest, authorities raided a home in the town of Sunbury, evacuating residents during their search. Officials didn’t say what they were looking for.

Residents were concerned that the evacuation might suggest there were explosives in the house or extremists living on the property.

“I was like, ‘Oh my God, what the hell is going on?'” one resident said.

The property is registered to foster parents who have been honored for their work with children. It’s not clear if the 18-year-old suspect had lived there.

Authorities increased the terrorism threat level to “critical” late Friday, after a bomb partially exploded during the morning rush hour.

Police are combing through closed-circuit TV images and have extensively studied the remains of the device without giving details about it. But images from inside the subway car after the blast showed that the device was contained in a bucket with wires hanging out of it and that it was concealed in a plastic shopping bag.

The train hit by the bomber at Parsons Green station in southwest London had video cameras in each car, and the London Underground network has thousands of cameras at the entrances to stations and along the labyrinth of subterranean and above-ground passageways leading from the entryway to the trains.

Officials have hinted there may be more than one person involved, but haven’t released details in what is termed an ongoing and covert inquiry.

Prime Minister Theresa May said raising the threat level to its highest point was a “proportionate and sensible step.” Police called on the public to be vigilant.

“This mean that their assessment is that a further attack may be imminent,” May said.

Security Minister Ben Wallace also warned there are “very dangerous individuals” on the loose that Scotland Yard is aware of.

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“Nearly 600 live investigations covering over 3,000 people and another group of 20,000 people we’re concerned of,” he said.

The soldiers will add to the armed police presence Saturday at public places to deter attacks after the Friday morning rush-hour blast on a District Line train. No arrests have been made. The explosion and an ensuing stampede at the station injured 29 people. None of the injuries, some of them burns, were believed to be life-threatening.

The bomb went off around 8:20 a.m. Friday as the train, carrying commuters from the suburbs — including many school children — was at Parsons Green station.

The station was reopened Saturday, officials said, restoring some normalcy to London’s transport network after a day of severe disruption. There was no sign of panic among Londoners and the weekend life of the city continued undeterred by the raised threat level.

Officials said the bomb was intended to do grave harm to commuters. Analysts said the injuries would have been far worse had the entire device exploded.

“They were really lucky with this one. It could have really become much worse,” said terrorism specialist Magnus Ranstorp of the Swedish Defense University.

The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit. The group also warned that a number of explosive devices had been planted, or would be planted, CBS2’s Jessica Borg reported.

The case, though, is unique. It doesn’t bear the group’s trademark martyrdom tactics.

“There are some odd aspects to this case. We have not seen a bucket before. It is a fairly crude device in a bag, left,” CBS Security Consultant Richard Walton said. “In recent times, attacks of this nature have been suicide bombers using backpacks.”

Sources close to the investigation believe this was probably the work of a lone wolf, Borg reported.

Here at home, NYPD Commissioner James O’Neill tweeted photos of what appeared to be the white bucket and bag inside the London subway train and said the NYPD is closely monitoring the incident.

O’Neill said while there are no direct threats to the city, New Yorkers should “always remain vigilant” and aware of their surroundings.

“In New York we live with the reality that we are the most targeted city in the United States by terrorist plots,” John Miller added.

President Donald Trump also took to Twitter Friday morning. He said, “Another attack in London by a loser terrorist” and suggested the terrorists were known to British police.

“These are sick and demented people who were in the sights of Scotland Yard,” he said. “Must be proactive!”

In a second tweet, Trump said: “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!”

Britain has endured four other attacks this year, which have killed a total of 36 people. The other attacks in London — near Parliament, on London Bridge and near a mosque in Finsbury Park in north London — used vehicles and knives.

In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people. That attack in May also briefly caused the threat level to be set at “critical.”

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)