Sen. Rand Paul (R-Ky.) said in an interview with Fox News Wednesday that arming “100 percent” of American pilots is the most “cost-effective” way of preventing terrorist attacks similar to those that occurred on 9/11.

“I’m concerned about what is the most cost-effective way of preventing another 9/11: I want all pilots to be armed,” Paul said on the Fox News program "Hannity," while discussing a bill to streamline pilots' carry permit process. “The president has zeroed this out of his budget. He’s advocated for getting rid of the program. And when I talk to pilots, I’m at airports all the time, pilots come up to me all the time and say it’s too hard to get a permit and to keep up the permit.”

After the Sept. 11 attacks, the Transportation Security Administration, under the directive of the Homeland Security Act of 2002, developed the Federal Flight Deck Officer program (FFDO), permitting pilots of commercial airline flights to carry firearms.

The following year, former President George W. Bush expanded the program to include pilots who fly all-cargo aircrafts.

Paul confronted former Homeland Security Secretary Janet Napolitano about the program's funding last April, characterizing the federal government’s “lack of commitment to the idea of self-defense” as a “huge signal to terrorists around the world if we're not going to arm our pilots.”

“Well, I’ll tell you the reason why we zeroed it out, Senator, and that is -- and it goes to a lot of the changes in the budget, we’re moving to risk-based. And an FFDO program is not risk-based,” Napolitano said during a 2013 Senate Homeland Security and Governmental Affairs Committee hearing. “It’s just happenstance, where you happen to have a pilot on board that went through the training or not. We’re offering the training to air carriers if they want their pilots covered. But we would rather stick with the FAMS [Federal Air Marshal Service], who are portion-based on risk.”

In 2008, a U.S. Airways pilot's gun accidentally discharged during a flight from Denver to Charlotte. The gun fired as the pilot -- who was part of the FFDO program -- was stowing the weapon, leaving a hole in the cockpit wall and an exit hole in the plane's exterior below the cockpit window.

Although the incident left the 124 passengers and five crew members unharmed, air safety experts warned that had the bullet pierced a window at a higher altitude, the shot could have caused the plane to rapidly depressurize.