Kent Police on Saturday arrested the first suspect, an 18-year-old in Dover, in connection with the bombing on an underground train in London. Twenty-nine people were injured in the explosion on Friday. Metropolitan Police Deputy Assistant Commissioner Neil Basu called the arrest significant, reported the BBC. “This arrest will lead to more activity from our officers,” he said.

The Islamic State had said they were behind Friday’s attack.

Earlier, Prime Minister Theresa May called the strike cowardly and raised the threat level in the United Kingdom from severe to critical, which means authorities believe that another attack is imminent. The terror threat level still remains at critical.