Three men were arrested in the south of Manchester on suspicion of being connected to the attack that hit a concert in the city, British police said Wednesday.

On Monday evening, a 22-year-old suicide attacker detonated a bomb as fans were leaving an Ariana Grande concert at Manchester Arena, killing 22 people, including children, and injuring at least 64, many of whom were battling life-threatening injuries.

"Three men have been arrested after police executed warrants in South Manchester in connection with the ongoing investigation into Monday night's horrific attack at the Manchester Arena," said Greater Manchester Police in a statement.

Interior minister Amber Rudd told press that it was likely that the assailant, Salman Abedi, was not working alone and that he was up to a point known to British secret services.

 The U.K. has raised its terrorism threat level to critical in the aftermath of a bombing at the Manchester Arena on Monday night, which left 22 people dead and 59 wounded. ISIS claimed responsibility for the attack, which targeted the attendees of an Ariana Grande concert—many of them teens and preteen girls.

The motive behind the bombing remains unknown, but Grande’s career thus far has illustrated the values of independence and freedom of expression that terrorism seeks to undermine, as reported in The Atlantic.

As terrorist attacks hit the European continent over the past year, Britain had, until last night, remained largely unscathed. Terrorists, these attacks suggest, are innovating; now, governments need to keep pace.

Prime Minister Theresa May said Tuesday that the country's terror threat alert had been moved up to its highest level, meaning an attack could be imminent.

As a result, Wednesday saw hundreds of troops deployed through the UK, standing guard at key locations in the largest cities.

The defense ministry said the change of guard ceremony at Buckingham Palace and tourist visits to Parliament would be canceled until the threat level was lowered.

Investigations are ongoing to discover potential motives behind the attack, which has been claimed by the Islamic State terror organization in an unverifiable message.