Just over a week ago airstrikes were shaking this city and hundreds of US Marines were sprinting through the streets. Now, the same Marines are handing out fliers and copies of the constitution, urging Iraqis to vote. But residents still don't know where they can cast ballots just two days before the country's crucial referendum. Across Iraq's insurgent heartland - the overwhelmingly Sunni Arab province of Anbar - fears of attacks have led to plans for only a handful of bunkered polling stations. The polling sites are still unannounced, officials said, and a ban on vehicular traffic will force many residents to walk miles to cast ballots on the constitution. US forces were not the only ones waging a public campaign: Insurgents had distributed fliers of their own before the US-led offensive, Iraqi soldiers said, threatening that anyone who votes will be beheaded. All these factors could depress turnout in a region key to whether the constitution is accepted or rejected. The US military hopes Sunni participation will bring those who feel marginalized into the political process and chip away support for the insurgency.