British police say they have made an arrest in connection with Friday’s rush-hour bomb attack on a London subway train.

Authorities say the 18-year-old man was arrested in the port area of Dover Saturday morning. Dover is a major ferry port for travel between Britain and France.  A police statement called the arrest “significant.”

Earlier Saturday London Transport authorities said they have re-opened the Parsons Green station where the bomb on a train partially detonated.

Images of the bomb posted on social media appear to show a bucket on fire that had been placed inside a plastic bag close to a railcar door.

Prime Minister Theresa May said after the attack that the country’s Joint Terrorism Analysis Center decided to raise the country’s threat level to critical, meaning that a further attack may be imminent.

ISIS has claimed responsibility for the assault. The terrorist group, however, has a history of claiming responsibility for attacks the group may not be connected with.

Shortly after the attack, armed police descended into the Parsons Green station.

May said the public may see more armed police on the streets and the transport network. The prime minister also said members of the military will begin aiding police, providing security at some sites not accessible to the public.

The National Health Service said late Friday that 21 people who were in the subway car at the time of the explosion were being treated at hospitals, while eight other people had been discharged.

The blast was the fifth major terrorist attack in Britain this year.

WATCH: Police Manhunt Following Terror Attack on London Underground

President Trump weighs in

U.S. President Donald Trump called Prime Minister May on Friday to convey his sympathies, the office of the White House press secretary said in a statement.

The statement said President Trump “pledged to continue close collaboration with the United Kingdom to stop attacks worldwide targeting innocent civilians and to combat extremism.”

Earlier, the British prime minister admonished Trump for his initial reaction to the attack. Trump had tweeted, “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!”

May responded to the tweet, telling the BBC, “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

​Police, intelligence agency work together

London police said their investigation into Friday’s attack is being supported by MI-5, Britain’s domestic intelligence agency.

British Foreign Minister Boris Johnson appealed for calm and said it was important not to speculate.

London Mayor Sadiq Khan said the British capital “will never be intimidated or defeated by terrorism.”
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