Monday’s terror attack at an Ariana Grande concert in Manchester, UK was carried out by a man who was known to British security services, and who was believed to be tied to a wider network of ISIS-linked operatives, according to a statement made by UK officials on Wednesday.

“It is very clear that this is a network we are investigating,” Greater Manchester Police Chief Constable Ian Hopkins said Wednesday. As a result of Monday’s attack and the ensuing investigation, the entire country has elevated its terror threat to critical, the highest the UK system allows, for the first time in more than a decade.

According to police, the bombing at Monday’s event at the Manchester Arena was carried out by Salman Abedi, a British-born national of Libyan descent.  Abedi was among the twenty-two-people killed by the blast, including children as young as eight years old.  British intelligence reports indicate that he had returned to the UK after a three-week trip to Libya only days before carrying out the attack.

Abedi was studying business and management at the University of Salford in Manchester.  Although he was considered “enrolled” for the current school year, he had stopped attending classes.