The TSA says that a new in-flight surveillance program, revealed by the Boston Globe on Saturday, is an important tool to keep U.S. skies safe that complies with privacy laws.

Under the program, called Quiet Skies, the agency began sending air marshals onto flights in March to monitor individuals who aren't under formal investigation and aren't on the terrorist watch list. 

"It's something that provides an ability to look at individuals who have exhibited patterns of travel that are not unlike what terrorists have exhibited in the past," TSA spokesman Jim Gregory said. "When those travel patterns along with other data are looked at, it trips an alert that places [individuals] into a category where there is a federal air marshal that might be on the flight."

While he didn't go into much detail about "other data," Gregory did say that it could include an individual's contacts and associations.

The program marks a significant shift for TSA, noted the Globe's report. Until March, the agency had only used air marshals on routes that were thought to be high-risk, or to monitor individuals on terrorist watch lists. 

The Globe story raised questions about the usefulness of the program, quoting communications among air marshals themselves, who lamented that they could be better deployed elsewhere. In one case, an air marshal was assigned to monitor a Southwest Airlines flight attendant. 

The Globe also quoted civil libertarians who wondered whether Quiet Skies, which monitors both U.S. citizens and foreign nationals, meets legal muster for privacy rights.

Speaking to Travel Weekly, Gregory said that the TSA didn't reveal the new in-flight surveillance program until it hit the press because the agency did not want to undermine its effectiveness.

"It is a sensitive program. Putting too much information out there about it provides an advantage to adversaries to potentially game the system," he said. 

Technically, Quiet Skies isn't new, having been implemented by the TSA in 2010. But until March, it involved only in-airport screening and observation of the identified subjects. 

Gregory said that the algorithm used by TSA to develop targets doesn't include race or religion, which is a concern of privacy advocates. The program, he said, has undergone a privacy impact assessment, as required under law. 

The Globe reported that the TSA surveils approximately 35 Quiet Skies passengers each day. Marshals monitor a variety of behaviors, including whether the subjects use the lavatory, fidget, use a computer, scan the boarding areas from afar, or wait to be last to board the plane. 

Individuals are taken off the Quiet Skies list after three months if they aren't found to be a risk, Gregory said. 

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