Those in custody had pledged loyalty to the Islamic State group and were seeking recruits to fight in Iraq and Syria, officials said.
"He won't go very far," President Beji Caid Essebsi says, while acknowledging the country faces a larger jihadist threat.
The attack on the Bardo National Museum that killed 25 was the deadliest terrorist offensive in the country since a 2002 suicide bombing.
Thousands who crossed Egypt's western border to find work have been returning home by land or by flights from Tunisia.
A suicide attack that killed at least 142 pulls Yemen deeper into a region-wide fight, but an ISIS claim is doubtful.
Two guards were having coffee, a third was eating a snack and a fourth never showed up — leaving no guards at Tunisia's museum and parliament.
Wednesday's attack in Tunisia heightened the European country's concern that it could be next.
A total of nine people were arrested Thursday in connection with the attack, during which five gunmen stormed the National Bardo Museum in Tunis. Twenty-three people were killed in the raid.
A major rally to mark the North African country's Independence Day is planned for Friday amid fears of follow-up attacks.
ISIS recently seized control of at least two cities along Libya's Mediterranean coastline after taking territory in Syria and Iraq last year.
Two of the gunmen were killed by security forces during Wednesday's attack on the Bardo Museum in Tunis.
Tunisia's newly elected government needs assistance to fight terrorists, and will likely get it.
A vital sector could face lasting repercussions from Wednesday's massacre at a national museum in Tunis.
Wednesday's violence could imperil the "tolerant and rights-respecting society" that Tunisians seek, a human rights group says.
The Islamic State group has claimed responsibility for the terrorist attack at a Tunisia museum Wednesday that killed 21 people and injured dozens more.
Tunisian authorities arrested nine people Thursday in connection with a terror attack at the National Bardo Museum in Tunis.
It's unclear what role the individuals had in connection with the attack at the Bardo museum Wednesday that killed 21 people.
The African Union said the attack at the Bardo Museum in Tunisia was "heinous and cowardly," and underlines the need for enhanced inter-African cooperation.
The death toll in the Bardo Museum attack risen to 23, including two dead gunmen. At least 18 foreign tourists were killed.
Officials have also listed South African, Italian, Spanish, German, Polish, Colombian and Tunisian nationals among the dead.
World leaders also condemn the violence that killed at least 20 in the country's capital and declare support for its citizens.
Social media accounts affiliated with the Islamic State group shared a message that seemed to foretell the attack that killed 20.