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 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 Government Accountability Office, the investigative arm of Congress, in testimony.

The findings, to be presented to the Senate Finance Committee, come as Congress considers delaying a 2007 deadline requiring passports or a small number of previously approved 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 sometimes cannot verify more than 8,000 different kinds of currently acceptable IDs without significantly slowing border traffic.

“This creates a security vulnerability we were hoping to close,” Agen said.

The Government Accountability Office probe follows a similar inquiry in 2003 and 2004 when undercover investigators crossed unhindered into the United States at least 14 times using counterfeit drivers’ licenses and, in one case, an expired, altered U.S. diplomatic passport. During that investigation, however, border agents in New York and Florida stopped three undercover officials who were using expired and forged passports, drivers’ licenses or birth certificates.

By comparison, between February and June 2006, 18 Government Accountability Office investigators breezed by border agents at checkpoints in California, Texas, Michigan, Idaho, Washington state, and twice each in Arizona and New York.

In two cases border agents did not ask the undercover investigators for any identification.

Two of the 9/11 hijackers used fake Virginia residency certificates to get valid state ID cards needed to board the planes that flew into the World Trade Center.

Neither Government Accountability Office probe specified the location of any border checkpoints investigators went through.

The 9/11 Commission called for tougher ID card rules at borders to help prevent terrorists from entering the country. In response, Congress in 2004 approved requirements for all travelers – including Americans – to show passports or a small number of other approved secure documents before entering the U.S.

Those requirements are supposed to take effect Dec. 31, 2007. But lawmakers from states that border Canada have since rebelled, contending the rules could hamper commercial and tourist travel. They are pushing to delay the rules by 17 months to ensure Homeland Security has proper technology to speed legitimate travel though border checkpoints that would minimize such hindrances.

Agen said Homeland Security agents intercepted 75,000 fraudulent documents from border travelers last year.

The department last month arrested a Mexican fugitive suspected of running a counterfeit document operation whose fake ID cards have turned up in all 50 states.