Update as of 7:20 a.m. EDT: The death toll in the attack on the Bardo Museum in Tunis has risen to 23, including the two dead gunmen, according to reports citing the health ministry. At least 18 foreign tourists, including one British citizen, three Japanese and two Polish nationals were among those killed, according to media reports.

Original story:

One of the gunmen who attacked the Bardo Museum in Tunis on Wednesday was known to the country’s intelligence services, Tunisian Prime Minister Habib Essid reportedly said Thursday, adding that his government is working with several other countries to find out more about the attackers. The attack, carried out by five gunmen on the museum connected to the parliament building, led to the deaths of at least 19 people, most of them foreign tourists.

In an interview with France's RTL radio, Essid revealed the names of the two attackers who were killed during the raid. However, it is not yet clear if the two men -- Yassine Laabidi and Hatem Khachnaoui -- were formally linked to a particular terrorist group. Essid reportedly said that though Laabidi had earlier been flagged by intelligence, it was not for “anything special.” Speaking after the attack, Tunisia's President Beji Caid Essebsi vowed to fight the militants "without mercy."  

“We should unite to defend our country. … These monstrous minorities do not frighten us. We will resist them until the deepest end without mercy. Democracy will win and it will survive,” Essebsi said, according to media reports.

The remaining three attackers have not yet been identified and are currently at large. Although no militant group has so far claimed responsibility for the attack, the raid came just two days after social media accounts linked to the Islamic State group claimed that an attack on the country was imminent.

“It appears likely that this was an attack by the Islamic State but we have to remember that there are also other possibilities,” Christopher Chivvis, a security expert at the RAND Corp., told CNN. “It could have been Ansar al Shariah in Tunisia, which is a local group. It could have been al Qaeda in the Islamic Maghreb.”

Of the 19 people killed by attackers, 17 have been identified as foreign tourists from Japan, Italy, Colombia, Spain, Australia, Poland and France. Two Tunisian nationals were also among those killed in the attack.