The gunman who killed 38 at a beach near the Tunisian city of Sousse had help in carrying out the attack, officials say.
Interior ministry spokesman Mohamed Ali Aroui said authorities were "sure" that Seifeddine Rezgui had had accomplices.
The government has announced increased security measures after the attack claimed by Islamic State (IS).
At least 30 of the dead were from the UK, the majority of those killed.
UK police said that 16 police officers had been deployed to Tunisia and hundreds more were working on the case in the UK in one of the largest counter-terrorism investigations since the 2005 London bombings.
Tunisian investigators believe the suspected accomplices provided the Kalashnikov assault rifle to Rezgui and helped him get to the scene, Mr Ali Aroui told AP.
He added that police were questioning Rezgui's father and three roommates, although he did not say any of them were suspected of involvement in the attack.
Neighbours and relatives in Rezgui's hometown of Gaafour have been telling journalists at their shock over his actions.
"Who could imagine he would commit such a horror?" uncle Ali Rezgui told Reuters.
"Maybe he was changed where he studied, maybe it was something on the internet. But we just don't have any answers," he added.
Tunisian authorities say army reservists will be deployed to tourist sites and that around 80 mosques accused of inciting violence will be closed within a week.
Friday's attack was the deadliest in Tunisia's recent history. In March, militants killed 22 people, mainly foreigners, at the Bardo museum in the capital Tunis.
Among those so far confirmed dead by family or friends are:
- Carly Lovett, 24, a beauty blogger and photographer from Gainsborough in Lincolnshire
- Sue Davey and her partner Scott Chalkley, both in their 40s
- Adrian Evans, 44, from the West Midlands and his 22-year-old nephew Joel Richards
One Belgian and one German have so far been identified among the dead, the Tunisian health ministry said.
One citizen of the Irish Republic is also confirmed dead. There were also thought to be Tunisians killed in the attack.
At least 36 people were injured, some seriously.
Tunisia's economy relies on tourism, and there are fears that the recent attacks will deal a major blow to the industry.
Security officials said Rezgui, who posed as a swimmer but was carrying a rifle under a parasol, started shooting on the beach before entering the Hotel Imperial Marhaba, continuing to shoot.
The country has struggled to contain a growth in militant Islamist violence since the overthrow of long-serving ruler Zine al-Abidine Ben Ali in in 2011.