A key aspect of Washington's aviation-security plan is seriously flawed, and the 39 managers of the country's largest airports have taken the bold step of saying so in a strongly worded joint letter sent last week to Secretary of Transportation Norman Mineta. A copy of the letter, obtained by TIME, bluntly takes on the new agency charged with fixing security, the Transportation Security Administration. The letter says the bomb-detection devices the TSA has ordered installed by Dec. 31 will create crowds of people in terminals who could be targets for attacks; the machines would be installed near terminal entrances and thus create huge congestion there. The devices would quickly become outdated, the letter adds, yet require big construction costs. The directors from airports including those in Atlanta, Dallas-Fort Worth, Denver, Miami, San Francisco and Washington say the TSA's one-size-fits-all security system cannot and should not be carried out in the next seven months. For the first time, they call on Mineta to stop or slow the process immediately by getting Congress to back off the deadlines. "Somebody needed to tell the truth," says Jeff Fegan, the director in Dallas and one of the letter's authors. Mineta's office says he has no intention of trying to change the law.