ISIS Claims Responsibility for Bardo Museum Attack

— -- ISIS today claimed responsibility for the Wednesday massacre at the Bardo Museum in Tunisia that killed 22 people, many of them Western tourists, and the two attackers.

In a 3 minute, 10 second audio message disseminated on twitter accounts associated with ISIS, the terror group said that the two dead gunmen, who it named Abu Zakaria al-Tunisi and Abu Anas al-Tunisi, “launched and were heavily equipped with machine guns and hand grenades to target Bardo Museum.”

“The blessed immersing operation led to killing and wounding dozens of Crusaders and apostates,” the message said, “and the failed security forces did not dare to approach but after the two heroes ran out of ammunition.”

ISIS also threatened more attacks to come, saying “what you have seen today is the first drop of the rain, Allah permitting. You will not enjoy security nor be pleased with peace while the Islamic State has men like these who do not sleep amidst grievances.”

The unverified claim, which is being analyzed by U.S. officials for authenticity, was promoted by "official" ISIS accounts "from a very early stage,” terror expert J.M. Berger, author of recent book “ISIS: The State of Terror,” and analyst for IntelWire, told ABC News.

"It definitely looks like an authentic claim; whether it's a truthful claim is a separate matter," Berger said.

ISIS’ claim came after Tunisian authorities said they had arrested nine people in connection with the attack.

“Four of the arrests were directly related to the attack, and five others were made under strong suspicion of relation to the attack,” Aida Klibi, a spokeswoman for the Tunisian presidential office, told ABC News about the arrests made Wednesday.

She would not reveal further information on the identity of those arrested.

Officials this morning said at least 24 died, and 48 more were injured, including tourists from Poland, Germany, Spain and Italy, most of them massacred as they got off a tour bus that brought them to the museum from a cruise ship.

Tourists inside the crowded museum in Tunisia's capital city ran for their lives, some shielding their children, as two gunmen approached. Later, they hid inside the galleries beneath priceless antiquities.

Two Spanish tourists and a guard were found still hiding today in the sarcophagus room, a museum official said.

ISIS has made repeated threats against Tunisia. In one video posted online in December, ISIS fighters urged Tunisian Muslim to pledge allegiance to ISIS and its leader Abu Bakr al-Baghdadi, and warned Tunisians “you will not live secured as long as Tunisia is not governed by Islam.”

Tunisian Prime Minister Habib Essid had earlier today said on French radio that one of the two gunmen was Yassine Laabidi, who was known to intelligence services but had no links to a particular terror group, according to the Associated Press.

ABC News' Michael S. James, Cho Park, Louise Dewast, Alexander Hosenball, Divya Kumar, and Phoebe Natanson contributed to this report.