Tunisian authorities are searching for potential accomplices in connection with Friday's mass shooting at a beach resort that left 38 people dead.
Interior Ministry spokesman Mohamed Ali Aroui told reporters Sunday that while a single Tunisian militant carried out the attack, officials are sure he had help beforehand.
Aroui said the gunman's cell phone has been found and his family has been questioned.
Many of those killed were British nationals, while others were citizens of Germany, Ireland, Belgium and Portugal.
British Prime Minister David Cameron wrote Sunday in the Daily Telegraph newspaper that Friday's attack in Tunisia, along with a mosque bombing in Kuwait that killed 27 people and a deadly assault on a chemical plant in France are part of an "evil" seen in other militant attacks.
"But we will not be cowed," Cameron said. "To our shock and grief we must add another word: resolve. Unshakeable resolve. We will stand up for our way of life."
The prime minister called on the world to better respond to terrorism, including boosting police and security services, countering online militant propaganda, and supporting governments in places like Iraq, Syria and Libya where extremists have taken advantage of turmoil.
Tunisia is mobilizing 1,000 extra police officers to tourist sites and beaches by Wednesday, and Prime Minister Habib Essid said about 80 mosques known for radical preaching would be shut down.
The Islamic State group claimed responsibility for the Tunisia attack, as well as the mosque bombing in Kuwait.
Authorities in Kuwait identified a Saudi national as the suicide bomber. They said he flew into the country early Friday hours before the attack. Authorities have detained the driver of the vehicle who drove the suspected bomber to the mosque, as well as the owner of the house where the driver was staying and five other people.
Reuters reported Monday that the Islamic State group released an audio recording featuring the bomber criticizing Shi'ites, "especially those in Kuwait," and hinting that soon there would be "blood" and "death."