An internal 2004 newsletter says an NSA office outfitted a 62-foot long airship with gear to collect information about shipping and had plans to use the balloons to carry out various eavesdropping missions.
The newsletter provides few details about the balloon tests but hints at expansive plans for using them to collect information.
"It's got dual airbags, three engines and one of the most sophisticated audio systems around," the post reads. "No, it's not the latest sports car from Europe — it's Hover Hammer, a steerable airship that may become one of the [multi-intelligence] platforms of the future."
It was launched from "an airfield near Solomons Island" — probably Naval Air Station Patuxent River.
The NSA did not immediately respond to questions about whether the balloons were still operating or whether it tests other intelligence gathering tools in American skies.
The military has made extensive use of balloons to carry out surveillance in recent years. The test at APG was designed to see if radar assemblies mounted on 243-foot blimps and then sent aloft could be used to spot cruise missiles being launched at the East Coast and coordinate with air defenses. After one of the balloons broke free of its moorings and floated into Pennsylvania, the program's funding was cut.
Before the test began, civil liberties campaigners raised concerns that the balloons could be used to spy on people on the ground using powerful cameras. The Army said it had no interest in doing so.
More recently, people in Baltimore were watched from the air under a secret trial program carried out for the city Police Department. A company called Persistent Surveillance Systems flew planes with special cameras that allowed investigators to track criminal suspects.