Fox News: Police officers in a small Arizona border city are on heightened alert following a tip that a Mexican drug cartel will put them in its crosshairs if they conduct off-duty busts. The threat stems from a marijuana seizure made this month by two off-duty police officers riding on horseback in an unincorporated area east of Nogales, a city of roughly 20,000, Police Chief Jeffrey Kirkham told FoxNews.com.