A car bomb exploded during Friday prayers at a mosque in Baghdad, killing eight people and wounding 20, police said. The blast came as two militant groups claimed responsibility for shooting down a Russian-made helicopter carrying 11 civilians, and one released a video purportedly showing insurgents shooting the crash's lone survivor. In another development, a third Iraqi militant group threatened to kill three kidnapped Romanian journalists and their Iraqi-American translator unless Romanian troops leave Iraq within four days, Al-Jazeera reported yesterday while airing a video of the captives. The video showed the three Romanians — two men and a woman — sitting cross-legged against a wall with their hands chained. The woman, sitting in the middle and said to be Marie Jeanne Ion, is seen talking but Al-Jazeera did not air the audio. Elsewhere, one US soldier was killed yesterday by a roadside bomb near the northern city of Tal Afar, the military said. On Thursday, a US Marine died in a nonhostile incident at Camp Delta, near Karmah, west of Baghdad, the military said. It gave no details. The violence was part of a surge of dramatic attacks that have caused heavy casualties in recent weeks, ending a relative lull since Jan. 30 elections.