The student who massacred holidaymakers on a Tunisian beach and at a hotel acted alone during the attack but had accomplices who supported him beforehand, an interior ministry official said yesterday.
Police were searching nationwide for more suspects after the slaughter of at least 38 people, including three Irish holidaymakers, in Sousse on Friday.
The 24-year-old killer's father and three roommates were detained and questioned in the capital, Tunis.
Attacker Seifeddine Rezgui, who was shot dead by police, was a graduate of Tunisia's Kairouan University where he lived with other students. The massacre was claimed by Islamic State.
"We are sure others helped, but did not participate," said a ministry spokesman.
Investigators believe the suspected accomplices provided the Kalashnikov assault rifle to Rezgui and helped him get to the scene.
A security official said Rezgui had frequented an "unofficial" mosque in the Tunisian holy city of Kairouan for the past two years.
He said a swimmer had found Rezgui's mobile phone in the sea. It showed he spoke with his father just before the attack.
The aftermath of Seifeddine Rezgui's crazed rampage at the beach resort of Sousse, captured on video from Tunisia TV1
Friday's attack on the Imperial Marhaba Hotel shook the North African nation that relies heavily on tourism and has struggled since its 2011 revolution to be the one Arab Spring country that succeeds in transitioning from authoritarianism to democracy.
Rezgut methodically moved from the beach to the hotel's swimming pool, reception and other areas.
The death toll surpassed the 22 people killed in March at The National Bardo Museum just outside Tunis - again mostly tourists - in a country known for its beaches and rich history.
Interior minister Mohamed Najem Gharsalli announced the deployment of 1,000 extra police officers at tourist sites and beaches.
"We don't want to make tourist establishments into barracks. That's not our goal. But we must act to guarantee the security of the tourist sector," he said.
It was not clear if the reinforcements would all be in uniform. There is currently a tourism police unit in holiday areas of Tunisia and numerous police wear civilian clothes.