"Gunmen in military uniforms attacked a museum in downtown Tunis around noon on Wednesday, killing 19 people" � including Polish, Italian, Spanish and German tourists.

"The identity and motivation of the attackers were not immediately clear."

Tunisia was the country where the Arab Spring revolts against autocratic rule began four years ago.

Of all the countries affected, Tunisia has made the most successful transition toward democracy, recently completing presidential and parliamentary elections and a peaceful rotation of political power. Security forces have struggled against occasional attacks by Islamic extremists, but they have usually occurred in mountainous areas far from the capital.