Seifeddine Yacoubi Tunisia Isis
Seifeddine Rezgui shot and killed 39 people, mostly Britons, before being killed in Sousse, Tunisia Facebook

The mastermind of the Sousse massacre, the worst terror attack on British citizens since 7/7, has been identified as Chamseddine al-Sandi, the head of a Tunisian Islamic State (Isis/Daesh) terror cell, now believed to be in Libya.

In documents obtained by the BBC, al-Sandi is linked to Seifeddine Rezgyui who opened fire on a beach near the Imperial Marhaba Hotel killing 38 people in June 2015. Thirty of those killed were British.

Al-Sandi ran the cell, known as the Bardo gang, responsible for the slayings at the Bardo Museum in Tunis in March 2015, in which 24 people were killed, as well as the Sousse attack.

Confessions obtained by investigators have shown that Rezgyui, who carried out his attack alone, worked closely with the Bardo gang. He trained in Libya alongside one gunman who would go on to carry out the attack at the museum.

Al-Sandi is reported to have recruited Rezgui and the gang, paid for them to be smuggled into Libya and go for training. Later he orchestrated their movements and gave them orders.

Al-Sandi, about who little else is known, is now in hiding in Libya. Tunisian authorities had already issued an arrest warrant for the jihadi prior to his identity becoming known to the media.

An inquest into the deaths of the British tourists in Tunisia is due to begin at The Royal Courts of Justice in London next week (16 January). However, the British lawyer representing the families has said he is unaware of al-Sandi's involvement.

Tunisia Sousse beach attack
People pray in a circle around flowers placed in memory of the victims murdered by Seifeddine Rezgui on the beach of the Imperial Marhaba resort in Sousse, Tunisia Reuters

Victims of the attack in Sousse have complained they were not given sufficient advice about security in the North African nation following the Bardo Museum attack.

They told the BBC that Thomson Holidays, which organised many of the bookings, assured its customers that heightened security would be in place following the upswing in violence in Tunisia.

TUI, the travel company that operates Thomson, has said in a statement that it wants to understand the circumstances that led to the attacks.

"We are cooperating fully with the coroner and will continue to do so, in order to help ensure that the tragic deaths of those killed can be thoroughly investigated, the relevant facts determined and any lessons learned," TUI said.