TUNIS, Tunisia -- Eight people are in custody in Tunisia, suspected of having direct links to a deadly beach attack that killed 38 people, but four other possible suspects have been released, a minister said Thursday.

The British Foreign Office, meanwhile, raised the death toll of British tourists killed in Friday's attack from 27 to 30. Other European tourists were among the dead. The attack was Tunisia's deadliest ever and threatens to be a devastating blow to its tourism sector.

In Britain, bereaved relatives looked on at an air base in Oxfordshire as a military plane flew home the remains of nine Britons on Thursday, a day after another flight returned eight other bodies.

The Islamic State group claimed responsibility for the attack, in which Tunisian student Seifeddine Rezgui opened fire on a beach in the resort of Sousse. He was later killed by police.

Government minister Kamel Jendoubi told reporters Thursday in Tunis the eight still detained -- seven men and one woman -- are suspected of direct links to the attack. He did not elaborate on their identities or roles, saying only that the investigation "has allowed us to discover the network behind the operation in Sousse."

The minister also urged greater international terrorism co-operation in a "war ... between democratic Tunisia and an international jihadi movement."

Sofiane Selliki, an official in the prosecutor's office, said no one had been brought before a judge for charges.

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

More Tunisians -- about 3,000 -- are believed to have gone to Syria and Iraq to join radical jihadis including the Islamic State group than fighters from any other country.

British vacationers made up the majority of the 38 killed on the beach, and further repatriation flights are expected in coming days.

Foreign Secretary Philip Hammond said earlier that all 30 British victims have been identified. Package holiday firms Thomson and First Choice confirmed that all were their customers.

A minute's silence in memory of the victims will be observed Friday, a week after the rampage, and flags will be flown at half-staff over London's Whitehall and Buckingham Palace.

Jill Lawless and Sylvia Hui in London contributed to this report.