Gunmen killed 19 people and sent tourists scrambling for cover in a siege at a museum in Tunisia's capital on Wednesday.The North African nation's Prime Minister called it a cowardly terrorist attack and warned that three suspects were still on the loose.Tunisian security forces killed two attackers as they ended the hostage siege at the Bardo Museum in Tunis, Prime Minister Habib Essid said. But the death toll, which included 17 tourists and at least one Tunisian security officer, could climb. Polish, Italian, German and Spanish tourists were among those killed, Essid said, with another 20 foreign tourists and two Tunisians wounded in the attack. "It's a cowardly attack mainly targeting the economy of Tunisia," the Prime Minister said. "We should unite to defend our country."The scene played out at a popular tourist destination in the heart of Tunisia's capital in a building linked to where the nation's parliament meets.