DUBAI - Masked men threw firebombs at an intelligence service compound in the city of Qatif in eastern Saudi Arabia, activists said on Monday, in an apparent reprisal for the execution of a prominent Shi'ite Muslim cleric earlier this month. A Saudi Interior Ministry spokesman said 'there was a failed terrorist attempt to burn the building with Molotov cocktails' and that one of the assailants was captured. Video footage posted on social media and dated Jan. 9 showed several masked young men moving under cover of darkness and lobbing firebombs over the protective outer wall of a building compound. Most of the firebombs were seen exploding on the ground inside, setting a nearby tree on fire. The authenticity of the footage could not immediately be confirmed. It was not initially clear who was behind the attack. But the recording contained a footnote indicating that it was carried out by Shi'ite youth seeking to avenge the execution of Sheikh Nimr al-Nimr, a vocal critic of the kingdom's Sunni Muslim monarchy.