Four people have been arrested by the Tunisian army in connection with Wednesday’s attack on the National Bardo Museum in Tunis that killed 21 people, British news outlet ITV reported Thursday, citing the office of Tunisian President Beji Caid Essebsi. So far, no terrorist groups have claimed responsibility for the incident, the deadliest attack on civilians in Tunisia in 13 years, according to the Guardian.

Two gunmen, identified as Yassine Laabidi and Hatem Khachnaoui, were shot dead by authorities shortly after the attack. Laabidi was no stranger to Tunisia’s intelligence community, but Tunisian Prime Minister Habib Essid said Laabidi was not on security forces’ radar for “anything special,” the Guardian reported.

It’s unclear what the roles were of the four people arrested in connection with the incident in which at least 18 foreign tourists were killed. Twenty-three people in all were slain, including the gunmen.    

Essebsi said Tunisia, viewed as the most stable country that experienced the Arab Spring from 2010 to 2012 in North Africa and the Middle East, would not be deterred by the attack at the Bardo museum. “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,” he said.

The attack spurred the Tunisian army to beef up security across the country, the government said in a statement.  "After a meeting with the armed forces, the president has decided large cities will be secured by the army," the statement read, according to ITV.

The museum is located adjacent to the Tunisian parliament, which was evacuated when the gunmen opened fire at the cultural institution, the BBC reported. The tourists killed in the attack hailed from Japan, Italy, Australia, Colombia, Spain, Poland and France, according to Essid. U.S. first lady Michelle Obama, who was in Japan during the attacks, expressed her condolences "the loved ones of those who were lost here in Japan and around the world."