Security experts for a major airline's pilot's union have warned members that potential terrorists conducted apparent "dry runs" aboard domestic flights in recent weeks, and urged flight crews not to be pressured into taking to the skies if they are fearful.

A memo from the U.S. Airline Pilots Association, which represents more than 5,000 pilots who fly for US Airways, cites "several cases recently throughout the (airline) industry of what appear to be probes, or dry runs, to test our procedures and reaction to an in-flight threat."

"Bringing down an airliner continues to be the Gold Standard of terrorism," states the undated memo, first reported by WTSP-TV in Tampa-St. Petersburg. "If anyone thinks that our enemies have “been there, done that” and are not targeting U.S. commercial aviation -- think again."

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On a Sept. 2 flight from Reagan National Airport in Washington to Orlando, a "Middle Eastern" man rose from his seat and sprinted toward the cockpit, before veering sharply to go into the forward restroom, according to the memo. While he was in there, sever other men moved about the cabin, changing seats and going into overhead bins, it says.

US Airways and the Transportation Security Administration confirmed the incident. Four passengers aboard the flight were detained by local law enforcement authorities upon arrival in Orlando due to suspicious behavior during the flight, according to a statement by Michelle Mohr, a spokeswoman for US Airways.

The TSA said in a statement the incident was checked out thoroughly.

"The Transportation Security Administration (TSA) takes all reports of suspicious activity on board aircraft seriously,” TSA spokesman Ross Feinstein told the station in a statement. “Working collaboratively with local law enforcement, it was determined that the matter required no further investigation at this time. Additionally, due to the critical mission of TSA’s Office of Law Enforcement, TSA does not confirm the presence or identity of Federal Air Marshals.”

According to the memo, the pilot of a subsequent, return flight bearing the same flight number, 1880, ordered an inspection of the plane after eight women in burkas showed up at the boarding gate. The memo claims that "evidence of tampering was found," though it does not elaborate.

One expert speculated that the efforts may have been aimed at sizing up security procedures and spotting air marshals aboard flights.

Rep. John Mica, R-Fla., has since called for an investigation into the “suspicious” incident.

“It is government’s obligation and responsibility to remain vigilant,” Mica said in a statement. “While the specifics of the US Air incident are not public, federal authorities must review the matter.”

Experts said the incident could be an indication of another attempt to detonate a bomb aboard an airplane while midflight. Ret. Col. Mike Pheneger, former director of intelligence at Special Operations Command, said it’s “impossible” to absolutely prevent terrorist attacks.

"We can only make it more difficult for people to attack an airplane or a facility,” he said. “We can't make it impossible. We have to be lucky 100 percent of the time and they only have to be lucky once."

Despite the constant threat, Pheneger said the odds of being on a plane taken over by terrorists are slim.

"But somebody is eventually going to be unlucky, and that will happen,” he said. “And I'm surprised quite frankly it hasn't."

One federal air marshal, meanwhile, told the station that if an attempt to blow up a plane developed mid-flight, it would likely lead to gunfire. As part of their training, air marshals are instructed that terrorists’ top priority is to identify and kill the agents who operate independently without backup and rank among those federal law enforcement officers who hold the highest standard for handgun accuracy.

"If the bad guys are there and they do what they're going to do, our job is to kill them," he said.

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