(CBSNews.com) - CBS News has learned the Federal Air Marshal Service or FAMS, whose agents travel undercover on U.S. domestic and international airplanes to prevent terrorist attacks, is closing six of its field offices.
According to an internal email sent to staff on Friday by Federal Air Marshal Director Robert Bray, the San Diego and Tampa field offices will close by the end of 2014. The Pittsburgh and Phoenix field offices will close by June 2015. And, the Cleveland and Cincinnati field offices are slated to close by June 2016.
The email also says the Las Vegas, Seattle, and Denver offices will be "assessed regularly, from the perspective of risk, intelligence, and industry trends" and "staffing levels in these offices will not be increased."
While the government will not disclose how many air marshals there are nationwide for security reasons, there are currently 26 field offices located in cities across the country near most major airports.
In a statement to CBS News, the Transportation Security Administration or TSA said the closures are "part of efforts to reallocate our workforce to allow the most effective security in the most efficient manner." The TSA also said "no positions are being eliminated and this will not adversely impact the ability of the FAMS to maintain coverage aboard flights arriving from and departing for the affected airports."
In his staff email, Bray said a budget reduction from $966 million to $805 million over the past three years has led to the "implementation of a number of efficiency measures" and the "decision is risk-based and intelligence driven." He promised that air safety would not be compromised.
But John Adler, the president of the National Federal Law Enforcement Officers Association which represents many air marshals, is concerned.
"It creates an unnecessary air travel vulnerability that may create opportunities for criminals to exploit," Adler said.
Adler has received numerous calls from air marshals whose anxiety about the news he says "is off the charts." Even though the TSA says no employees will lose their jobs Adler believes some air marshals may not be able to relocate their families to other cities.
"They are concerned because there is a lack of communication and clarity of what's going to," Adler said. "And, they are concerned there is nothing to back fill the missions they provide and what the consequences could be to the public."
© 2014 CBS Interactive Inc. All Rights Reserved.