Joel Rosenberg, a firearms instructor in Minneapolis, pulls back his coat to display his gun outside the property of Minneapolis-Paul International Airport on Wednesday in Minneapolis. In some of the nation's busiest airports, including Minneapolis, it's perfectly legal to carry a loaded gun right up to security checkpoints.

Atlanta ? Flying in the U.S. has been transformed since Sept. 11, with passengers forced to remove their shoes, take out their laptop computers and put liquids and gels in clear plastic bags. Yet it is perfectly legal to take a loaded gun right up to the security checkpoint at some of the U.S.’s biggest airports.

An Associated Press survey of the 20 busiest U.S. airports found that seven of them – Philadelphia, Detroit, Phoenix, Minneapolis/St. Paul, Dallas/Fort Worth, Los Angeles and San Francisco – let people with gun permits carry firearms in the general public areas of the terminal.

Some anti-terrorism experts say that is a glaring security loophole that could endanger airport workers, passengers and people waiting to pick them up or see them off. Some suggest that allowing guns in terminals is practically asking for them to be smuggled aboard a plane.

“If your airport is not secure, then the security of your airplanes is jeopardized,” said Rafi Ron, former security chief at Ben Gurion Airport in Israel who now works as an aviation consultant. “You cannot separate the two.”

Other authorities say the nonsecure areas of the terminal are no different from other public venues and do not warrant special restrictions.

“It’s really not more of a concern than at a mall or a train station,” said Philadelphia police Lt. Louis Liberati.

Under federal law, it is illegal everywhere to try to carry a gun through a security checkpoint. The rest of the terminal, however, has long been the domain of state and local authorities.

Jon Allen, a spokesman for the federal Transportation Security Administration, said the TSA has not taken a position on guns in airports and has no authority under federal law to ban them.

The issue has led to a clash in Georgia between a new state law that allows guns on public transportation and the Atlanta airport’s ban on loaded weapons. Last month, a federal judge dismissed a lawsuit brought against the city by a gun rights group. At an earlier hearing, he warned that guns at the world’s busiest airport could pose a “serious threat to public safety and welfare.” The gun group has appealed.

Rep. Bennie Thompson, a Democrat and the chairman of the House Homeland Security Committee, was surprised to learn that airports have been allowing weapons for years, and warned that Congress could move to ban the practice. In a July letter to TSA, Thompson called guns in terminals “a threat to the safety of airline travelers.”

However, even at those airports that ban guns, officials are not frisking people or using metal detectors on them as they enter the terminal. Experts say an additional layer of security like that would be unworkable at America’s bustling airports.

Gun rights supporters say law-abiding citizens with guns could fire back and cut short a gunman’s rampage.