This is according to a GAO report.

The number of inadmissible aliens who managed to enter through official ports of entry in 2006 was not disclosed in Monday’s report from the Government Accountability Office.

However, a source who has seen a full version of the report, in which those statistics were included, put the total at 21,000.

The author of the GAO report, Richard Stana, said most of those who were wrongly allowed to enter were economic migrants who did not present a security risk.

“But as we saw in the recent past, it doesn’t take too many people getting through the ports of entry to cause some real trouble,” he said. “And not everyone who comes in and is a danger needs to be a terrorist. It could be someone connected with a criminal enterprise.”

Understaffing and turnover at Customs and Border Protection, the agency that oversees the nation’s 326 land, sea and air ports of entry, has contributed to the problem, according to the GAO report. However, investigators also cited weak management controls and complacency and inattentiveness by some officers.

GAO investigators arriving at one point of entry found no border agents in the inspection booth, while at other locations, agents didn’t ask for travel documents, according to the report.

The story notes that about 400 million people enter the US legally every year, so by comparison, 21,000 doesn’t seem like a lot. But that’s 21,000 who, if they felt like it and had the means, could cause an awful lot of damage. And it’s 21,000 that the CBP is supposed to keep on the other side of the borders and checkpoints.