A third attacker in the deadly assault on the Bardo museum is on the run, Tunisia's president said Sunday, declaring his country at war with the extremists who killed 21 people at one of North Africa's most revered cultural institutions. President Beji Caid Essebsi said the attack involved "three aggressors" and the third man escaped. He spoke live with French network iTele from inside the museum, its elaborate tilework visible behind him. Tunisia's Interior Ministry released security camera footage of Wednesday's attack showing two gunmen walking through the museum, carrying assault rifles and bags. At one point they encounter a third man with a backpack walking down a flight of stairs. They briefly acknowledge each other before walking in opposite directions. "Definitely there were three," Essebsi told France's Itele television and Europe 1 radio, according to the French news agency AFP. "Two were killed, but there is one who is now on the run," he said. "In any case, he will not get very far." Police responding to the attack shot and killed the two gunmen. They were identified as Tunisians in their 20s who had trained in Libya. Essebsi said the extremists who have recruited about 3,000 Tunisians to fight in Iraq and Syria have no credible connection to Islamic belief. He said his country was at war with them. "When war is brought upon us, we will wage war," he said. Essebsi said that a monument would be built to commemorate the dead. The gunmen killed 21 people in an attack Wednesday at the National Bardo Museum in Tunis, all but one of them foreign tourists. A Tunisian policeman died in the attack. Some material for this report came from AFP and Reuters.