Secret Service agents, TSA screeners, air-traffic controllers – and oil and gas permit processors.

As some 800,000 essential federal workers on Friday went without paychecks and people across the country clamored for the government to reopen from its ongoing shutdown, a handful of bureaucrats were among those back at work approving drilling applications for the oil and gas sector – a move that some say is illegal and possibly even criminal.

The Bureau of Land Management, which oversees energy production on federal land, has opened offices in New Mexico and on Monday plans to reopen four more offices in Wyoming to approve oil and gas drilling applications – even as other businesses that depend on access to federal public lands remained shuttered and conservation and environmental groups say their calls to bureau offices went unanswered.

A spokeswoman with the Bureau of Land Management did not immediately confirm the plan to open additional offices in Wyoming, which was first reported by The Casper Star-Tribune in a story citing a memo from the Petroleum Association of Wyoming. The trade group had apparently pushed the bureau to reopen the offices despite the shutdown.

"We just wanted to understand if it's happening elsewhere, why it isn't happening here," association President Pete Obermueller told the Star-Tribune. "It turns out, it took a little bit longer, but we're just glad they followed suit here."

Wyoming last year was the eighth-largest producer of oil in the U.S.; New Mexico was the third-largest. Oil production on federal lands, meanwhile, has soared.

The news that the Bureau of Land Management offices had reopened – while other offices and services remained shuttered amid the shutdown – sparked shock and outcries from conservation and environmental advocates.

"We're a conservation organization. We interact with BLM on a regular basis, and we cannot interact with them right now – they won't answer their phones, they're not in their offices. How is it the oil and gas industry is reaching them?" says Jeremy Nichols, director of the Climate and Energy Program for WildEarth Guardians. "We're getting nothing but silence from the bureau, and the oil and gas industry is apparently getting 100 percent of their attention."

WildEarth Guardians, in a series of tweets , contended that the work is illegal. The organization pointed to the Anti-Deficiency Act , which prohibits voluntary work during a shutdown except "for emergencies involving the safety of human life or the protection of property."

"Is there an emergency that requires them to process these drilling permits? I cannot fathom how an emergency possibly exists," Nichols says. "The real emergency is the state of our public lands – there's nobody watching them now. This, to us, is an incredibly corrupt double-standard that's playing out here."

Whether the work being performed is legal remained in question. Many details – from how many employees were performing the work to whether they were being paid – were also unclear. It is possible that the work is being paid by the fees companies pay with their drilling applications.

"It would appear that that is illegal, and technically it's something that would be subject to criminal sanctions," says Cary Coglianese, a law professor at the University of Pennsylvania Law School who studies government regulation and environmental law. "Of course that would require someone from the Justice Department initiating a criminal action, which it's hard to imagine they would do that."

Oddly, one of the only ways that the work may pass legal muster is if the employees are somehow the only ones – out of roughly 800,000 furloughed federal workers – being paid during the shutdown. While the Anti-Deficiency Act bars the federal government from receiving most voluntary services during a shutdown, it doesn't stop the government from paying for certain services if it's got the money.

The Nation’s Capital: Shutdown, D.C. View All 29 Images

In a "contingency plan" for a shutdown published in last March, the Bureau of Land Management seemed to hint at this possibility, stating that employees "working on selected energy, minerals and other associated permit activities for which the Bureau charges a processing fee" would be considered "exempt" and required to continue to reporting for work.

"The question is, who's paying the guys who are doing the work? That becomes a complicated budgetary question whether or not it's a designated fee," says Brendan Collins, a partner in the environment and natural resources group at Ballard Spahr. "This presents a very curious situation. The House may well be inclined to investigate this further as to whether or not this represents the proper disposition of funds."

House Democrats on Friday did not sound eager to look into the issue. One committee staff member indicated that Democrats did not plan to challenge the legality of opening the Wyoming and New Mexico offices.

Environmental groups, however, were outraged.

"It's not at all reasonable to completely shut the public out from any access to agencies like BLM while allowing private industry – the oil and gas industry – to have apparently full access to the agencies," says Connie Wilbert, director of the Sierra Club Wyoming Chapter. "The public has no way to get access to the agency to find out what's going on, yet right in our local newspapers, the Petroleum Association of Wyoming is saying, 'We're working with BLM to get this stuff open again.'