The National Park Service might have violated federal law and its own regulations when it allowed Washington Redskins owner Daniel Snyder to cut more than 130 mature trees from a slope behind his Potomac estate last fall, attorneys for two environmental organizations said this week.

The Park Service never assessed potential environmental damage, as required by law, before issuing Snyder a permit to clear 50,000 square feet of forested land on a protected easement he owns along the Chesapeake & Ohio Canal, according to attorneys for the Audubon Naturalist Society and the Potomac Conservancy.

Both organizations are calling for an investigation by the Interior Department, which manages the Park Service.

"It's a little baffling these regulations that are used on a daily basis by the Park Service in just about everything they do were somehow conspicuously ignored," said Meredith Lathbury, an attorney for the Potomac Conservancy.

Under the National Environmental Policy Act, passed by Congress in 1969, federal agencies must consider the environmental impact of proposed activity on federal land. For the Park Service, that means producing an environmental assessment and giving public notice -- neither of which occurred in the Snyder matter.

In October, the Park Service told Snyder, who said he was trying to rid his property of non-native species, that he would have to conduct an environmental assessment before any agreement could be finalized. But the Park Service issued the tree-clearing permit anyway, allowing Snyder to improve his view of the Potomac River, Park Service documents show.

"I think there was an end run on NEPA and public participation, and in my mind this raises very serious questions," said Jim Jamieson, an attorney for the Audubon Naturalist Society.

Kevin D. Brandt, superintendent of C&O Canal National Historical Park, who negotiated the deal with Snyder last fall, said he was acting on the advice of regional Park Service officials. "I am not an expert in all these fields," he said.

Brandt's comments conflict with earlier statements that he alone made the decision allowing removal of the trees.

A spokesman said Snyder was unavailable to comment yesterday.

Bill Line, a spokesman for the Park Service's National Capital Region, said the government issued the permit legally under an exemption in the Environmental Policy Act. Line directed a Washington Post reporter to language in the permit that said requests involving clearing of non-native species can go forward without environmental review.

"We don't believe we violated either the spirit or the letter of the law," Line said.

The Park Service provides for other instances where environmental review is not required if no "measurable impacts to the human environment" exist, according to agency regulations.

One exception deals with removal of non-native species that "pose an imminent danger to visitors."

Lathbury, noting Snyder was also allowed to cut 20 native species, said the exemptions should not have applied.

"There is an impact to scenic resources and water quality, and they have to look at that," Lathbury said.

The Park Service allowed Snyder to cut the trees in exchange for a possible share of the enhanced value he would receive from his new view.

Snyder was allowed to remove trees from an 8.3-acre easement the government purchased in the 1970s.

The initial easement, which officials said was outdated because it allowed Snyder to remove underbrush, was replaced with one giving Snyder future control over the height of trees closest to his house while barring him from touching vegetation closest to the canal.

The agreement called for an appraiser to inspect Snyder's $10 million estate and determine its value with the new view.

At the same time, the Park Service would calculate the value of additional easement protections it received from Snyder, along with the value of about 1,300 native saplings and small trees that he planted. If the value of the enhanced view exceeded the value of what the Park Service gained, Snyder would donate the difference to the government, under the agreement.

Montgomery County officials are investigating whether Snyder violated local conservation laws by removing the trees without approval from the Maryland-National Capital Park and Planning Commission.

Last month, Brandt said he had thought Snyder had the county's permission. Brant, who said he now feels misled, said the deal is on hold pending the outcome of Montgomery's investigation.