Some 21,000 people who should not have been allowed to enter the U.S. came through official border crossing points between Oct. 1, 2005 and Sept. 30, 2006, according to a government report released Monday.

After the 2001 terrorist attacks, the government reorganized its border security operations and increased security measures to prevent people from falsifying travel documents or using other deceptive methods to enter the country through such legal entry points as airports and border crossings. Some of the hijackers who carried out the Sept. 11 attacks entered the U.S. this way.

The Government Accountability Office found that Customs and Border Protection officers turned away 200,000 people who tried to enter the country through the 326 legal air, sea and land entry points during the 2006 fiscal year. The numbers do not include people who unlawfully entered the country through other routes.

The GAO's findings are based on a statistical estimate of the number of people who could have passed through the entry points, Customs and Border Protection's deputy commissioner, Jayson P. Ahern, said Monday. In reality, the government does not know exactly how many people passed through checkpoints who should not have been allowed in the country.

"The point is that we need to actually do a better job," Ahern said. Customs, as mandated by Congress, has stepped up efforts to make sure everyone who enters the country has a valid document, he said, and about 400 million people enter the U.S. through legal checkpoints each year.

Staffing shortages and poor management at legal border crossings are among the reasons that people got through improperly, GAO found. A publicly released version of its report states "several thousand" of these people made it past Customs and Border Protection officers. An official with access to more detailed information told The Associated Press the number is about 21,000.

The chairman of the House Homeland Security committee said these numbers should be a wake-up call for the Homeland Security Department, which oversees border entry operations. "As we continue to pump more resources into virtual and real fences between our ports of entry, we cannot afford to lose sight of other vulnerabilities at our borders," Rep. Bennie Thompson, D-Miss., said in a statement Monday. Thompson referred to the millions of dollars going toward a fence along the southwest border to prevent people from illegally entering the U.S. between controlled crossing points.

But James Carafano, a security expert at the conservative Heritage Foundation think tank, said fixing the problems pointed out in the GAO report is not a cure-all.

"It's a stupid way to look for terrorists," Carafano said, adding that a better system would look for terrorists not only at the border, but also at points long before they arrive there. Policing only legal entry points is "never going to catch everybody," he said.

Lee Hamilton, the vice chairman of the independent commission that investigated the 2001 terrorist attacks, said the GAO's findings represent a huge failure in the system. But Hamilton said people are always going to be able to slip through.

"You've got millions of people crossing these borders, and perfection is beyond reach," he said.

The Homeland Security Department has come under recent criticism because two men with a dangerous strain of tuberculosis were able to cross into the United States this year through legal entry points, despite instructions not to let them in.

While these men are not among the 21,000 cited in the report released Monday, they represent an ongoing problem in applying rigorous new screening techniques. There are weaknesses in the government's other screening methods as well. An October GAO report, for example, found that the terrorist watch list is not used as consistently as it should be.