WASHINGTON (AP) — The FBI assured Congress in an unusual, confidential briefing that its plane surveillance program is a by-the-books operation short on high-definition cameras — with some planes equipped with binoculars — and said only five times in five years has it tracked cellphones from the sky.

The FBI would not openly answer some questions about its planes, which routinely orbit major U.S. cities and rural areas. Although the FBI has described the program as unclassified and not secret, it declined to disclose during an unclassified portion of a Capitol Hill briefing any details about how many planes it flies or how much the program costs. In a 2009 budget document, the FBI said it had 115 planes in its fleet.

The briefing Wednesday to Senate staff was the first effort in recent years — if ever — to impose oversight for the FBI’s 30-year aerial surveillance program that gives support to specific, ongoing investigations into counterterrorism, espionage and criminal cases and ground surveillance operations. While it withheld some details, it offered assurances that the planes are not intended to perform mass surveillance or bulk intelligence collection. However, there is still no formal oversight regimen for the program.

The briefing came two weeks after the FBI confirmed to The Associated Press for the first time its wide-scale use of the aircraft, after the AP traced at least 50 planes registered to fake companies back to the FBI. The AP investigation identified more than 100 flights in 11 states over a 30-day period this spring. The planes since June 1 have flown more than two-dozen times over at least seven states, including parts of Texas, Georgia and the Pacific Northwest.

The ubiquity of the flights, combined with few details about the surveillance equipment aboard the planes, raised civil liberties concerns over Americans’ privacy.