The Islamic State claimed responsibility Thursday for an attack on a popular museum in Tunisia that left 23 people dead a day earlier, according to an audio recording posted online.
Thursday's statement described the attack as a "blessed invasion of one of the dens of infidels and vice in Muslim Tunisia," and appeared on a forum that carries messages from the group, The Associated Press reports.
The statement said there were two attackers and they weren't killed until they ran out of ammunition and it promised further attacks.
"Wait for the glad tidings of what will harm you, impure ones, for what you have seen today is the first drop of the rain," the statement, which was also announced by U.S.-based SITE Intelligence Group.
US intelligence officials confirmed to Fox News that they are aware of the tape and are reviewing it.
There is no reason to doubt the authenticity of the ISIS claim, the officials said.
ISIS, which is based in Syria and Iraq, has affiliates in neighboring Libya, where many Tunisians have gone to fight and train with extremist groups.
Twitter accounts associated with the Islamic State terror group based in Syria and Iraq were described as overjoyed at the attack, urging Tunisians to "follow their brothers," according to Rita Katz of SITE.
ISIS is not believed to have a foothold in Tunisia, but has established a presence in neighboring Libya and Algeria. A counterterror analyst told Fox News that the museum attack may be linked to the death of Ahmed Rouissi, a senior ISIS commander in Libya whose body was found last week near the city of Sirte. Rouissi was a leading member of Tunisia's Ansar al-Sharia group and believed to be responsible for the killing of two Tunisian opposition leaders in 2013.
Earlier this week, a prominent Tunisian field commander for ISIS was killed in fighting inside Libya.
Earlier Thursday, Tunisian police arrested nine people in connection with the violence, sources in Tunisia told Fox News.
A statement from Tunisia's presidential office said that five of those arrested were directly connected to the operation, which involved two gunmen attacking the National Bardo Museum in the country's capital, Tunis. Four other suspects were linked to the attackers and were based outside the capital.
Newly-elected Tunisian President Beji Caid Essebsi said Wednesday that Tunisia was "in a war with terror." He condemned what he described as "savage minority groups" after gunmen stormed the museum.
Hours after the police ended the siege, thousands of Tunisians flocked to downtown's landmark Bourguiba Avenue, where the 2011 revolution took place, for a nighttime rally. They chanted for a "Free Tunisia" in defiance of terrorism.
"I want the people of Tunisia to understand firstly and lastly that we are in a war with terror, and these savage minority groups will not frighten us," said Essebsi in a televised address to the nation. "The fight against them will continue until they are exterminated."
The deadliest attack on civilians in the North African country since 2002 began early Wednesday afternoon when two militants wearing military-style uniforms and wielding assault rifles burst from a vehicle and began gunning down tourists climbing out of buses at the museum.
The attackers, identified as Yassine Laabidi and Hatem Khachnaoui, then charged inside to take hostages before being killed in a firefight with security forces. Prime Minister Habib Essid said that said Laabidi had been flagged to intelligence, although not for "anything special."
Security forces guarded major thoroughfares Thursday while authorities hunted for two or three accomplices believed to have been involved in the attack.
One cruise ship whose passengers had been among the victims pulled out of the port of Tunis early Thursday. MSC Cruises said nine passengers from the Splendida were killed, 12 were injured and six were unaccounted for. Another ship, the Costa Fascinosa, said 13 passengers had not returned on board when the ship left port overnight.
Essid said the gunmen killed 18 tourists total -- including four from Italy, two from Colombia, two from Spain, and one each from Australia, Poland and France. Japan’s government said three of its citizens were killed.
Five Tunisians also were killed, including two attackers. Health Minister Said Aidi said all the injuries came from bullet wounds, and that several victims were brought in without identity documents.
At least 44 people were wounded, including tourists from Italy, France, Japan, South Africa, Poland, Belgium and Russia, according to Essid and doctors from Tunis' Charles Nicolle hospital.
Witnesses described the gunmen "shooting at anything that moved" inside the museum, which is Tunisia's largest.
Dozens of tourists scrambled from the museum, linking arms or clutching children as Tunisian police and security forces pointed their weapons at the building. The museum, about 2 miles from the city center, is located near the national parliament building, which was evacuated.
South Carolina native Carol, who asked that her last name not be used for safety concerns, told Fox News that she and a friend hid in a museum alcove during the attack, which saved her life.
“All of a sudden we heard bang, bang, bang, and people were running out the door en masse and our tour guide left at the end of the group who were running," she said. My friend and I stayed behind…neither of us like to get into crowds…and all of a sudden we were alone in the museum!”
Carol said she saw bodies of two people who had rushed out of the museum in the first group.
“I am very thankful that we got out safely,” she told Fox News. “Whether we would have, if we had exited with the group or not, that is the unknown.”
A Spanish man and a pregnant Spanish woman who had been inside the museum during the attack hid in the building all night in fear and were retrieved safely Thursday morning by security forces, Aidi told The Associated Press.
The United States, France, the United Arab Emirates and the United Nations denounced the bloodshed. U.S. Secretary of State John Kerry said Washington "condemns in the strongest possible terms today's deadly terrorist attack" and praised Tunisia's "rapid response" to resolve the hostage situation and restore calm.
Fox News' Greg Palkot, Catherine Herridge and The Associated Press contributed to this report.