Tunisian Interior Ministry spokesman Mohamed Ali Aroui said authorities were "sure" that Seifeddine Rezgui had accomplices who provided him the Kalashnikov rifle, BBC reported on Monday.
Security officials said Rezgui, who posed as a swimmer, was carrying a rifle under a parasol and started shooting on the beach before entering the Hotel Imperial Marhaba.
On Sunday, officials confirmed that out of the 38 victims, 30 were from Britain.
British police have said that 16 police officers have been deployed to Tunisia and hundreds more were working on the case in Britain in one of the largest counter-terrorism investigations since the 2005 London bombings.
The Tunisian government has announced increased security measures after Islamic State (IS) claimed responsibility for the attack by deploying army reservists at tourist sites.
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.
The city of Sousse, 140 km south of capital Tunis, is one of the most prominent tourist destinations in the country.