WASHINGTON — They say U.S. critter assassins work in secret, quietly laying traps, lacing food with poison, sniping at targets from helicopters. Few people know exactly how the hits go down; the methods are largely hidden.

What’s certain is that the little-known U.S. Department of Agriculture Wildlife Services kills up to 3 million animals a year, mostly those deemed a nuisance but also some that agents kill by mistake, including endangered species.

Now, in a turnabout, the hunter is the target. A petition seeks to reduce the power of Wildlife Services and shine a light on its practices, claiming its agents have “gone rogue,” overstepping the mission to protect the public by killing indiscriminately.

There’s no dispute that Wildlife Services plays a valuable role by eliminating invasive animals such as nutria and starlings that are a menace. But critics have questions: How many is too many? Does the agency euthanize wildlife too often on behalf of farmers and ranchers without regard to ecosystems?

The petition filed early this month by the Center for Biological Diversity isn’t the first time that animal rights activists have squared off against Wildlife Services, but this time their coalition includes politicians who agree that the agency is too secret and too deadly. Even some federal workers frown on it; staff members at the U.S. Fish and Wildlife Service quietly dismiss Wildlife Services agents as “gopher chokers.”

“Wildlife Services is one of the most opaque and obstinate departments I’ve dealt with,” said Rep. Peter DeFazio, D-Ore. “We’re really not sure what they’re doing. I’ve asked the agency to give me breakdowns on what lethal methods they’re using. They can’t or won’t do that. We’ve asked them to tell us what goes into their poisons. They won’t say.”

DeFazio and several colleagues requested a congressional hearing on the agency’s practices without success, so they pushed the USDA inspector general, Phyllis Fong, to conduct an audit, which was announced this month.

Wildlife Services said in response that it has nothing to hide. Answering questions by email, a spokeswoman said that the bulk of its work is to protect humans.

“For example, we work with the aviation community to protect the public by reducing wildlife hazards at more than 800 airports around the country,” spokeswoman Lyndsay Cole said. “Wildlife Services’ efforts to protect threatened and endangered species are conducted in more than 34 states. Wildlife Services also operates the National Rabies Management Program, which distributes oral vaccines in 16 states.”

The service has consulted with wolf management agencies to . . . “lessen the negative impacts of expanding wolf populations since the 1970s,” she said. In the Rocky Mountains and Great Lakes regions, “the wolf population has reached and exceeded recovery goals” set by managers, she said.

Cole said the agency’s kills are guided by a science-based decision-making model. In a 1997 review, its most recent, the agency described it as “the integration and application of all practical methods of prevention and control to reduce wildlife damage.”

“This basically says they can use whatever methods at their disposal, whenever they want,” said Amy Atwood, a senior attorney for the Center for Biological Diversity. “Just saying you comply with the law doesn’t make it so.”

Concerns over Wildlife Services spiked last year when agent Jamie Olson posted photographs on Facebook showing his dogs attacking and mauling a coyote caught in a trap, said Camilla Fox, founder and executive director of the nonprofit group Project Coyote.

Olson acknowledged he made a mistake, “not in his coyote hunting practices but in letting the photos, which he says are more than five years old, be publicly accessible,” according to a blog post in the weekly Missoula (Montana) Independent.

In January, Wildlife Services trapper Russell Files was arrested in Arizona for intentionally snaring a neighbor’s dog in a steel trap. Before it was rescued, the animal lost 17 teeth trying to chew off its leg.