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.

“The intelligence services know a lot of people, and I’m sure we will find out more what level they knew about (Abedi) in due course. But at the moment all they have confirmed is that they did know about him. And as I say, we will find out more when the operation is complete,” French Interior Minister Gérard Collomb told BFM-TV.  She also confirmed that Abedi had “proven links to ISIS.”

Libya has become a hot bed for terrorism since the downfall of dictator Muammar Gaddafi in 2011.  The following year, an attack on an American diplomatic center in Benghazi, Libya resulted in the death of Ambassador Chris Stevens, as well as Sean Smith with the U.S. Foreign Service, and Navy SEALS Glen Doherty and Tyrone Woods.

British Prime Minister Theresa May raised the nation’s terror threat level from “severe” to “critical” on Tuesday night, nearly twenty-four hours after the explosion tore through the arena.  She also warned the nation that a “further attack may be imminent.”  This development in conjunction with claims of a wider network operating within Europe with ties to the culprit may indicate that the British investigation has uncovered plans for subsequent attacks, or at least that Abedi’s accomplices remain active within the UK.

Three men were arrested in South Manchester on Wednesday, and at least one other was detained on Tuesday as the investigation continues to develop.  Another man was taken into custody for wielding a knife near Buckingham palace on Wednesday, but the arrest was ultimately ruled unrelated to the terrorist incident.