Federal air marshals are surveilling thousands of ordinary U.S. citizens through a Transportation Security Administration program called Quiet Skies, the Boston Globe reported.

How does this happen?

When someone is placed under surveillance, a group of armed, undercover air marshals is assigned to his or her flight, according to the report. The air marshals proceed to spy on them, write minute-by-minute reports and log the information with the TSA, according to information reported by the Globe.

Typically, the people being monitored are travelers who are not under investigation by any agency, nor are they in the Terrorist Screening Database, according to TSA documents obtained by the news outlet.

In addition to monitoring international travelers, the Globe reported:

The teams track citizens on domestic flights, to or from dozens of cities big and small — such as Boston and Harrisburg, Pa., Washington, D.C., and Myrtle Beach, S.C. — taking notes on whether travelers use a phone, go to the bathroom, chat with others, or change clothes, according to documents and people within the department.

Additionally, a person’s identification could be considered suspect if his or her appearance has changed. This includes common changes that can happen over the years such as an increase or decrease in weight, new hairstyles, or changes to a man's goatee, mustache or beard, according to the report.

All U.S. citizens re-entering the country are automatically screened for possible inclusion in Quiet Skies, the report stated. Travelers’ patterns and affiliations are checked. And their names are run through a terrorist watch list and “other databases,” according to documents obtained by the Globe.

An internal bulletin states the program’s goal as averting potential threats to commercial aircraft as “posed by unknown or partially known terrorists,” according to the report. But the terminology gives the agency a free reign to place anyone under under close scrutiny, the report noted.

Some air marshals told the Globe they are not comfortable with the program, which required them to monitor travelers who appear to pose no real safety threat.

The work is tedious and time consuming and takes away from actual law enforcement duties, the marshals told the news outlet.

What did the TSA say in defense of the practices?

In a written statement, TSA officials defended the program by telling the Globe it helps stop “potential acts of terror.”

“But the agency declined to discuss whether Quiet Skies has intercepted any threats, or even to confirm that the program exists,” according to the report.

Revealing the details “would make passengers less safe,” agency spokesman James Gregory told the Globe.