BAGHDAD, Iraq - A U.S. Army Black Hawk helicopter went down in northern Iraq, killing all 12 Americans believed to be aboard in the deadliest crash in nearly a year, while five U.S. Marines died in weekend attacks, the military said Sunday.The latest deaths followed an especially bloody week in which about 200 Iraqis and a dozen U.S. troops were killed. Iraqi politicians, meanwhile, claimed headway in forming a stable coalition government following the Dec. 15 elections, whose final results may be released this week. U.S. military officials said the UH-60 Black Hawk crashed just before midnight Saturday about seven miles east of Tal Afar, a northern city near the Syrian border that has seen heavy fighting with insurgents. "All (those killed) are believed to be U.S. citizens," military spokesman Lt. Col. Barry Johnson said.