British authorities arrested an 18-year-old man in Dover on Saturday as part of the ongoing investigation into Friday's train bombing in London, which left dozens of people injured.

“We have made a significant arrest in our investigation this morning,” said Neil Basu, the U.K.'s senior national co-ordinator for counter terrorism policing, according to Reuters.

Dover is port city in the southern part of the country, where people frequently use passenger ferries to sail to France.

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The arrest comes one day after Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE said the country's threat level had been raised to "critical," meaning an additional attack could be imminent.

British authorities previously raised the threat level to critical in May after a man killed 22 people at an Ariana Grande concert in Manchester.

Basu said the threat level would continue to remain at critical following Friday's bombing at London's Parson's Green train station, which left 30 people injured.

British authorities announced they were treating the incident as a terror attack.

President Trump was quick to condemn the attack on Twitter, saying it was carried out "by a loser terrorist."

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

May responded to Trump's characterization, saying it is never helpful for someone to speculate on an ongoing investigation.