ISIS apparently claims responsibility for Tunisia museum attack; 9 arrested

ISIS apparently claimed responsibility Thursday for the deadly terrorist attack at a landmark museum in the heart of that country's capital, a mass shooting that has shaken the birthplace of the Arab Spring and stirred questions about militants in the country.

In an audio statement posted online Thursday, ISIS identified two men -- Abu Zakariya al-Tunisi and Abu Anas al-Tunisi -- it said used "automatic weapons and hand grenades" to kill and injure what it called "crusaders and apostates" in the Bardo Museum in Tunis. Tunisian Health Minister Said Aidi said 23 people are believed to have been killed, including at least one who died at a hospital overnight.

And that bloodshed, the ISIS message warned, is "just the start."

CNN cannot independently verify the legitimacy of the audio statement.

A U.S. official told CNN there is no reason to doubt the claim's authenticity. That said, American officials are checking the platform that the statement went out on, including the extent to which it's tied to the group calling itself the Islamic State.

Gunmen attack Tunisian museum 15 photos

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The current U.S. thinking is the attack may have been carried out by local "franchise" adherents to ISIS, rather than centrally directed by the Islamist extremist group's leadership, which is now thought to be in Syria.

The two attackers were carrying explosives, Tunisian President Beji Caid Essebsi said in an interview Thursday with the French broadcaster TF1.

He credited Tunisian security forces for responding so quickly to the attack to avoid a larger tragedy because "terrible explosives were found on these (attackers) and they didn't have time to use them."

Tunisia has been viewed as the lone democratic success story in the Arab Spring. But the North African nation is not without its issues, including an uneven economy and the distinction of having more citizens -- up to 3,000 Tunisians -- thought to have gone to Iraq and Syria to fight as jihadists than any other country, according to the International Centre for the Study of Radicalization in London.