British Prime Minister Theresa May on Tuesday condemned the terrorist bombing in Manchester as an attack of “appalling, sickening cowardice, deliberately targeting innocent defenseless children and young people who should’ve been enjoying one of the most memorable nights of their lives.”

May said Britain’s terrorism threat level remained “severe,” meaning that attacks are “highly likely.” Counter-terrorism officials are assessing whether that threat level should be ratcheted up in the coming days.

The bombing at the Manchester Arena, following a concert by singer Ariana Grande, killed 22 people and injured 59. The attacker detonated his homemade bomb at about 10:30 p.m. Monday “near one of the exits of the venue, deliberately [choosing] the time and the place to cause maximum carnage and to kill and injure indiscriminately,” May said.

“It is now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our society with cold calculation,” May said, following an emergency meeting of government ministers Tuesday morning. “This is among the worst terrorist incidents we have experienced in the United Kingdom.”

While she confirmed that the bombing was the work of a lone attacker, authorities are trying to determine whether he was connected to a terrorist network. May said police had identified the bomber but were withholding his name for the time being.

Manchester police said later that a 23-year-old man had been arrested in south Manchester in connection with the attack.