Gunmen shot down a US attack helicopter during fighting in western Baghdad on Sunday, killing its two-crew members. Insurgents and Marines called a cease-fire in the besieged city of Fallujah, but the fragile peace was shaken by shootings that wounded another two Americans. Insurgents kidnapped an American civilian and killed a U.S. soldier in the area Friday. President Bush, attending an Easter service at a chapel at Fort Hood, Texas, braced the country Sunday for the possibility of more American casualties in Iraq while saying the US-led mission is just. Hundreds of US reinforcements moved in place on the city edge, joining 1,200 Marines and nearly 900 Iraqi security forces already involved in the fighting.