Undercover federal agents armed and trained to stop hijackings may be sitting in the seat next to you on your flight, but you'll never know unless there is trouble.

But whether there are enough sky cops to prevent disaster may also never be known, critics say.

"We are not asking them to tell us the flights. We're not asking them to tell us the numbers. We want an assurance that the law has been complied with. When they can't give us that assurance and they are withholding information, not because of a national security reason, but perhaps because they just don't have enough people," said Charles Slepian, an aviation security expert at the Foreseeable Risk Analysis Center.

When President Bush signed new airport security measures into law last November, sky marshals were ordered to take to the skies on domestic as well as international flights. The new legislation required undercover cops to be stationed on potentially high-risk flights, including non-stop, cross-country routes like those flown by the four planes hijacked on Sept. 11.

Travelers interviewed by Fox News at San Francisco International Airport said they would feel much safer knowing whether an air marshal is on their flight, but the number of active sky marshals and the planes on which they are flying has been classified by the Transportation Security Administration for reasons of national security.

It's that secrecy, Slepian said, that makes it difficult to know if the law is being followed.

"I would be satisfied if the TSA were to say to the public that what we promised you has been fulfilled -- that on every flight where there is a risk, there are adequate sky marshals to handle that risk," he said.

TSA officials told Fox News that there are thousands of sky marshals in the air, far more than before Sept. 11, and it's no longer accepting applications for the job.

But with 35,000 flights and millions of passengers a day, industry observers hope it doesn't take another disaster for the public to know for sure whether the air marshal program is working.

Fox News' Claudia Cowan contributed to this report.