About 12 people have been detained in the investigation into a massacre at a beach resort that killed 38 people, mostly British tourists, a Tunisian official said.

Tunisia's Radio Mosaique reported Thursday that Lazhar Akremi, a senior official with the ruling Nida Tounes party, made the announcement at a party meeting. The report did not elaborate on when or where the arrests occurred.

The Tunisian government is holding a news conference Thursday morning in the capital Tunis about the investigation. The Islamic State group claimed responsibility for Friday's attack, in which Tunisian student Seifeddine Rezgui opened fire on a beach in the resort of Sousse. The attacker was later killed by police.

Investigators are searching nationwide for accomplices. A person close to the investigation told The Associated Press on Monday that seven people had been detained in the probe.

At least 27 Britons and other European tourists are confirmed among the dead.

A top security official told the AP this week that the student had trained in a jihadi camp in Libya at the same time as the two men who attacked a leading Tunisian museum in March. That enforced the notion of a link between the two assaults and raised fears of more attacks on this North African nation's budding democracy.

The attack was Tunisia's deadliest ever, and threatened to be a devastating blow to the country's tourism sector, which is crucial to the economy and had just started recovering after uncertainty following Arab Spring uprisings.