At least 30 people out of the 38 killed at a popular resort in the Tunisian city of Sousse last Friday were British citizens, British media reports claimed Monday.

The identification process of victims is underway, but Tunisian law prohibits the release of victim names until a legally valid proof of identity is provided.

The Queen on Monday sent her "sincere condolences" to victims' families, saying she was "shocked" at the attack, Buckingham Palace said in a statement.

British Prime Minister David Cameron published an article in the U.K. Telegraph on Monday in which he vowed that the U.K. would show "unshakeable resolve" in dealing with extremism.

Meanwhile, security forces in Tunisia have launched a nationwide search for accomplices, who may have helped the gunman who carried out Friday's deadly attack.

A Tunisian Interior Ministry official told the press on Sunday that the gunman, student 23-year-old Seifeddine Rezgui, did not act alone. The attacker's father and three roommates have been detained and are being questioned in Tunis, the official said.

Tunisian police believe the suspected accomplices provided the Kalashnikov assault rifle to Rezgui and helped him get to the scene, the spokesman said.

The alleged gunman Rezgui had opened fire on tourists while they relaxed by the beach, reportedly shouting to people that he was out to kill only foreigners and not the Tunisian people. The attack continued until police eventually shot him dead Friday.

Anadolu Agency