One of the gunmen behind Wednesday’s attack at Tunis’ Bardo museum, which left 21 people dead, was already known to authorities, Tunisia’s prime minister has said.

Habib Essi told RTL Radio that Tunisian security services had flagged Yassine Laabidi as a potential danger, but they were not aware of any specific threats.

Seventeen tourists, including citizens from Britain, Japan, Italy, Colombia, Australia, France, Poland and Spain, and two Tunisians, including a police officer, were killed when two attackers stormed into the Bardo National Museum shortly after midday on 18 March.

The attackers targeted tourists as they alighted a bus to enter the museum. As the tourists fled inside the building to avoid the shooting, the attackers followed them and a three-hour siege ensued.

Two attackers were killed at the scene, and authorities are searching for others connected to the attack, including three suspected accomplices still at large.

The motivations for the attack are unclear and no group has claimed responsibility for the attack, but social media accounts connected to the Islamic State celebrated the incident urged Tunisians to “follow their brothers”.

Hundreds of Tunisians held a vigil to pay their respects to the victims and condemn the attack as cowardly on Wednesday evening.