Environmental groups and activists have sued to stop the “ill-considered” plan to sell Plum Island, charging the federal government failed to comply with five separate laws in advancing an auction of the 840-acre site.

The suit, filed Thursday in U.S. District Court in Central Islip, accuses the U.S. Department of Homeland Security and the General Services Administration of relying on a “fundamentally flawed” environmental impact statement that failed to properly explore preservation of the undeveloped portion of the island.

The groups have argued that the 2008 federal law that initiated the sale requires only the disposition of 170 acres tied to the Plum Island Animal Disease Center, not the undeveloped portion, which they want preserved.

Led by Save the Sound, the suit’s seven plaintiffs said they hope legal action will help preserve the island and surrounding waters, all of it teeming with wildlife, including endangered and threatened species. They want to preserve the 670 undeveloped acres as a “unique and extraordinary de facto nature preserve.”

“The federal government got it wrong” in auctioning the entire island, said Bob DeLuca, president of Group for the East End, at a gathering at Orient Point on Friday to announce the suit. “This [suit] is what it takes when government goes awry to set things straight.” The law firm Morrison & Foerster has taken the case pro bono.

Louise Harrison, a conservation biologist with the Plum Island Coalition, said a lawsuit was an unavoidable last resort.

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“Trying to save what we already own makes me very angry,” she said, pointing to the “dismissive, cavalier, unreasonable behavior” by the federal government. Homeland Security spokeswoman Gillian Christensen said the agency didn’t comment on pending litigation.

The GSA is leading the sale to recoup costs of relocating the research center to a new $1.25 billion facility in Manhattan, Kansas.

“We’re not opposing the sale of that [lab] portion of the island,” said Roger Reynolds, an attorney for Save the Sound. “What we’re asking is either to keep the other portion in federal hands as a wildlife refuge, or something like that, or to do a conservation sale.”

The suit charges federal officials violated the Endangered Species Act, among other laws, when they failed to consider a sale’s impact on natural resources, despite a chorus of protests from state, federal and local lawmakers.

Court papers also said the defendants failed to detail the cleanup required at the disease research lab. Critics had hoped such detail would dissuade potential buyers.

The plaintiffs include Peconic Baykeeper and Group for the East End.

Two other defendants were named: Denise Turner Roth, head of the General Services Administration, and Homeland Security Secretary Jeh Johnson, whose department runs the laboratory.

In the past, Homeland Security officials referred questions to the GSA, which had said it was committed to following the law.

The suit had been contemplated for more than a year as environmental groups awaited the outcome of congressional efforts on both sides of the Long Island Sound to stop the sale.

But a much-anticipated and much-criticized federal report released last week failed to address opponents’ numerous concerns, they said.

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With Gary Dymski