As Congress considers a new immigration law that would expand the fleet of unmanned drones along the border, the agency in charge of border protection is increasingly offering the military-grade drones it already owns to domestic law-enforcement agencies and has considered equipping them with "nonlethal weapons," according to documents recently made public.

The documents, which include flight logs over the past three years, were unearthed by the Electronic Frontier Foundation through a Freedom of Information lawsuit.

Agencies that have used the 10 Predator drones owned by the Customs and Border Protection Agency have deployed them to investigate fishing violations, search for missing persons and inspect levees along the Mississippi River, among other things. Three years ago, the drones were used by other agencies 30 times; in 2012, that jumped to 250 times. How the agency stores and shares that data with other government agencies remains unclear.

Additionally, the agency, in a 2010 report to Congress included in the documents, raised the possibility of eventually equipping its drones with "nonlethal weapons" to "immobilize" people and vehicles trying to cross the border illegally.

In a statement Wednesday, the agency said it had "no plans to arm its unmanned aircraft systems with nonlethal weapons or weapons of any kind." It said the drones supported the agency’s border security mission and provided "an important surveillance and reconnaissance capability for interdiction agents on the ground and on the waterways."

The drones, the agency said, "were designed with the ability to add new surveillance capabilities, accommodate technological developments, and ensure that our systems are equipped with the most advanced resources available."

The specter of drones in U.S. skies has been contentious, far more so than other common surveillance tools. Proponents of drones, including the military contractors who build many of them, say they can be useful for a variety of purposes, like monitoring crops and finding missing children, and a handful of police agencies have already bought small, lightweight unmanned vehicles that can fly for short bursts of time.

Skeptics say the use of drones raises the prospect of ubiquitous monitoring, especially by law enforcement, and several states have already proposed measures to restrict their use by police.

"What concerns me is the lack of clear, transparent rules for domestic drone use," said Rep. Zoe Lofgren, D-Calif., who recently introduced legislation to limit their use in domestic airspace. She said she was concerned about "the government’s increased interest in using drones for domestic surveillance and security, including the potential use of force. But the law today has weak requirements for individual privacy protection, transparency of drone use, and limitations on arming drones with weapons."

Lofgren was co-sponsor of a bill in the House this year that would, among other things, prohibit the use of firearms on drones in domestic airspace, although not necessarily other weapons like tear gas or pellets.

The agency has used Predator drones, the same vehicles used overseas by the U.S. military, since 2005. Built by General Atomics, the drone weighs about 10,500 pounds and can fly for 20 hours nonstop. They are based in Arizona, Florida, North Dakota and Texas.

"CBP needs to assure the public that it will not equip its Predators with any weapons – lethal or otherwise," Jennifer Lynch wrote for the Electronic Frontier Foundation, based in San Francisco, in a blog post Wednesday about the documents. "Without first addressing these issues, the agency — and Congress — should halt the expansion of CBP’s Predator drone program."

The flight logs provided by the agency show that it has become increasingly generous with its unmanned aerial vehicles. They have been used by the FBI, the North Dakota Army National Guard, Texas Department of Public Safety and the U.S. Forest Service, among others.

The use of drones by the FBI came to light only two weeks ago, when its director, Robert S. Mueller III, in response to a question, told the Senate Judiciary Committee that unmanned aerial vehicles were "very seldom used" by his agents. He went on to say that the agency was developing guidelines on their appropriate use. An earlier information request filed by the Electronic Frontier Foundation suggested that the FBI had at least three drones in its possession, as of 2011, when that request was filed.

The border agency said that when conducting joint operations with state, local and other federal agencies, its own privacy policies governed the use of data collected by the drones and "the live feed from any aircraft is encrypted and only accessible to those with specific clearance."

An audit by the Department of Homeland Security last year criticized the border agency for failing to put in place enough resources to maintain its drones and coordinate their deployment. At the time, the agency recommended holding off on any further acquisitions.

Nonetheless, the border agency now stands to increase its fleet. The Senate immigration bill, passed last week, authorizes the agency to buy four additional drones, along with new radar equipment.

Other documents obtained this year by the Electronic Privacy Information Center, based in Washington, had found that cameras attached to the Predators could distinguish between a human figure and a beast but did not use facial recognition technology.

Privacy advocates worry about the prospect of law-enforcement officials using drones to patrol particular areas for long stretches of time or follow particular people without a warrant.

"The danger comes from dragnet surveillance," is how Ryan Calo, a law professor at the University of Washington, described it. He called himself mostly a champion of drone technology, but with safeguards to protect privacy.

"By dragnet I mean indiscriminate," he said, "not in a particular situation, but just to buzz around looking for suspicious activity, which is exactly what they do on border."