Federal, state and local law enforcement agencies are increasingly borrowing border-patrol drones for domestic surveillance operations, newly released records show, a harbinger of what is expected to become the commonplace use of unmanned aircraft by police.

Customs and Border Protection, which has the largest U.S. drone fleet of its kind outside the Defense Department, flew nearly 700 such surveillance missions on behalf of other agencies from 2010 to 2012, according to flight logs released recently in response to a Freedom of Information Act lawsuit filed by the Electronic Frontier Foundation, a civil-liberties group.

The records show that the border-­patrol drones are being commissioned by other agencies more often than previously known. Most of the missions are performed for the Coast Guard, the Drug Enforcement Administration and immigration authorities. But they also aid in disaster relief and in the search for marijuana crops, methamphetamine labs and missing persons, among other missions not directly related to border protection.

Because they have sophisticated cameras and can remain in flight for many hours at a time, drones create novel privacy challenges. Civil libertarians have argued that these aircraft could lead to persistent visual surveillance of Americans on private property. Government lawyers have argued, however, that there is no meaningful legal distinction between the use of unmanned and piloted aircraft for surveillance.

The issue has become a hot topic in Congress; the Senate Commerce, Science and Transportation Committee is scheduled to hold a hearing on the subject Wednesday.

For now, drone flights in the United States are tightly restricted for safety reasons. Other than the military, Customs and Border Protection is one of the few agencies permitted by the Federal Aviation Administration to fly unmanned aircraft on a daily basis within the country’s borders.

As a result, Customs and Border Protection is facing heavy demand to fly its unarmed drones to benefit other law enforcement agencies that lack their own.