SAN DIEGO, California (CNN) -- Their mission is to protect airline passengers from acts of terror on U.S. flights. But in a special investigation, former and current air marshals told CNN that the number of marshals assigned to police flights is so low that the federal agency overseeing them has drastically lowered its firearms and psychological testing standards just so it can qualify new hires.

An air marshal, far left, trains during a simulated hijacking days after the 9/11 attacks.

More than a dozen current and former marshals said that so many federal air marshals have resigned and are not being replaced, airport screeners are being employed to fill the dwindling ranks.

But the Transportation Security Administration says that's not true and that the rate of those leaving has remained at 6.5 percent a year since 2001.

A former federal air marshal and weapons trainer who left the agency in 2006 after four years of service said the situation was so bad that managers at his office fudged the numbers by assigning marshals to short, no-risk flights.

The former marshal said that was done to make it appear that the percentage of manned flights was higher than it really was.

"I think it's a national disgrace,'' said the former marshal, who asked not to be identified because he still works in law enforcement. Watch Drew Griffin's investigative report »

The Federal Air Marshal Service was greatly expanded in the aftermath of the terrorist attacks of September 11, when flights to "high-risk cities" such as New York were given special air marshal manpower priority.

Assignments are "intelligence-driven" and "risk-based," the Federal Air Marshal Service said in an e-mail. But many of the marshals interviewed said it had little to do with intelligence or risk and was more about a numbers game.

"We were questioning how these flights could be intelligence-driven when we were flying from San Diego to Phoenix on another leg to Las Vegas back to Phoenix back to San Diego," the former marshal said. "It's not a threat flying on Southwest Airlines to Las Vegas."

Faced with fewer qualified applicants, current air marshals said that recruiting standards have been lowered. Air marshals still patrolling flights also said the loss of so many experienced agents has led the TSA to hire airport screeners as air marshals.

Agency spokesman Greg Alter said in an e-mail that only "a very small number of air marshals started their careers as Transportation Security Officers [airport screeners]."

Alter added that all "candidates receive the best training available and enter the workforce with the skill and expertise needed to protect the traveling public."

In July 2006, the Federal Air Marshal Service sent out a memo saying that new hires would no longer face mandatory psychological testing, unless the recruit admits that he or she has been treated for a mental condition. Read a copy of the memo »

TSA said it revised but did not "degrade" the psychological testing of applicants using the application and interaction with others in the service to determine mental competency.

On firearms training, a former weapons instructor with air marshals said that when recruits could not pass the tough federal tactical pistol course, known as the TPC, it was replaced with a less rigorous shooting test the potential recruits could pass.

"The TPC went away very quickly because they couldn't get enough people through it to pass," the former air marshal trainer said. "So they dropped the tactical pistol course and went to the practical pistol course, which is a standard federal law enforcement course. It's not nearly as quick or as dynamic as TPC."

But the TSA disputes the claim, saying it altered the weapons training six years ago because marshals needed more of a police-type training program rather than military-style weapons instruction.

The TSA said in an e-mail that "the course of fire and minimum qualification score air marshal candidates must acquire is the same today as it has been for over six years."

To replace departing air marshals, the TSA hired internally, including some administrative staff who had no college, law enforcement or military backgrounds, one current marshal said.

"To me, it's more of an embarrassment to be a member of that agency that would allow that particular individual in the training program," one marshal said. "I wouldn't want them on my flight. ... I don't want them as my partner."

The revelations come in the wake of a CNN investigation, in which air marshals and pilots said that only about 1 percent of the nation's 28,000 daily domestic flights were protected by onboard, armed federal marshals.

The Federal Air Marshal Service disputes that figure.

CNN's report about the declining number of marshals on planes also got the attention of Congress.

In a congressional hearing this week, the head of the Transportation Security Administration, Kip Hawley, told members of Congress that what CNN heard from the air marshals is wrong.

"I have to just correct on the factual basis on the CNN report about air marshals covering 1 percent. That number is absolutely wrong by an order of magnitude, and it was a guess by the folks there, and I just have to say that number is completely false."

Hawley would not say what percentage of flights has air marshals. That's a national security secret.

The service hides behind national security to keep the public from knowing how thin coverage really is, air marshals said.

The Federal Air Marshal Service continues to refuse CNN's request for an interview.

This month, Democratic Rep. Sheila Jackson Lee of Texas, who serves on the Homeland Security Committee, began holding closed-door meetings with the air marshal's service to determine whether congressional oversight committees are getting the truth.

"We will keep working and continuing to make sure that the airlines are served with the appropriate law enforcement that ensures the safety of the traveling public. We, too, are not interested in having funny numbers," Jackson Lee said.

Jackson Lee said that the committee has not finished its work and that she is convinced American air travel is safe for passengers. "It is important to restate and to re-emphasize: This is not an open opportunity for those who would attempt to do Americans harm. We are light years from where we were in 2000. We have trained personnel. They're being utilized, and we feel that we are steps ahead of where we were, but we want to get better. And that's what we intend to do."

After seeing CNN's initial report, Democratic Sen. John Kerry of Massachusetts sent a letter to Homeland Security chief Michael Chertoff asking for clarity on the number of air marshals protecting domestic flights and sought a response by April 11.

The senator is still waiting, Kerry's staff said. E-mail to a friend

Todd Schwarzschild also contributed to this report.

All About Air Travel • Terrorism • Transportation Security Administration