WASHINGTON - Undercover investigators entered the United States using fake documents repeatedly this year — including some cases in which Homeland Security Department agents didn't ask for identification. At nine border crossings, on both the Mexico and Canadian borders, agents "never questioned the authenticity of the counterfeit documents," according to Government Accountability Office testimony to be released Wednesday. "This vulnerability potentially allows terrorists or others involved in criminal activity to pass freely into the United States from Canada or Mexico with little or no chance of being detected," concluded the GAO, the investigative arm of Congress, in testimony obtained Tuesday by The Associated Press. The findings come as Congress considers delaying a 2007 deadline requiring passports or tamperproof ID cards from all who enter the United States. Homeland Security spokesman Jarrod Agen said agents are trained to identify false birth certificates, driver's licenses and other documents. But he conceded that agents are sometimes unable to verify more than 8,000 different kinds of currently acceptable IDs without significantly slowing border traffic.