Washington ? Terrorists carrying radioactive materials could easily enter the United States from Canada undetected, government investigators said Thursday after they were able to cross the 5,000-mile border four times carrying a large, red duffel bag without being intercepted.

The crossings took place at unguarded and unmonitored sites in four northern-border states. The Government Accountability Office, Congress’ investigative arm, did not disclose the sites.

“Our work shows that a determined cross-border violator would likely be able to bring radioactive materials or other contraband undetected into the United States by crossing the U.S.-Canada border at any of the locations we investigated,” the report said.

Even though the northern border is more than twice as long as the U.S.-Mexico border, it has less than one-tenth as many agents patrolling it.

Lawmakers from northern states blamed the White House and the Department of Homeland Security for the lax security on the U.S.-Canada border.

“This report is just as unacceptable as it is shocking,” said Sen. Patty Murray, D-Wash., who along with others have sought to secure more Border Patrol agents and high-tech surveillance and communications equipment for the northern border. “We simply cannot accept this level of vulnerability.”

The report said investigators found state roads close to the northern border that did not appear to be guarded or monitored. In three cases, investigators were able to cross from Canada into the United States undetected. In the fourth case, Border Patrol agents were alerted, but the investigators were able to disappear into the United States before the agents arrived.

In addition, the report said it found several ports of entry that were staffed during the day but not at night.

Though U.S. Customs and Border Protection (CBP) said such sites were equipped with surveillance equipment, GAO investigators spent 12 minutes at one of them taking pictures. Border Patrol agents never arrived. The GAO investigators were later stopped three miles from the site but were only briefly questioned, and their vehicle was not searched.

The GAO border-security report is available at http://www.gao.gov/new.items/d07884t.pdf