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With a record $13 billion sticker price, the settlement deal between JPMorgan Chase and the government captured the attention of Wall Street and Washington late last year.

And yet, according to a lawsuit that a nonprofit group filed against the Justice Department on Monday, the crucial details of the deal were for the government’s eyes only. The lawsuit filed by Better Markets, a group critical of Wall Street, challenged the constitutionality of the deal, a landmark settlement stemming from accusations that JPMorgan overstated the quality of mortgage securities it sold before the financial crisis.

In a complaint filed on Monday in the United States District Court for the District of Columbia, Better Markets argued that the Justice Department violated the constitutional principle of separation of powers when it “unilaterally” struck the deal without a judge’s blessing.

“The executive branch, through DOJ, acted as investigator, prosecutor, judge, jury, sentencer and collector, without any review or approval of its unilateral and largely secret actions,” Better Markets said in the lawsuit.

The lawsuit from Better Markets, the latest development in a case that was long thought to be closed, could provide a backdoor route for subjecting the deal to judicial scrutiny. Better Markets is seeking to have a judge approve an injunction that would scuttle the pact — unless the Justice Department provides “an ample and detailed record so that such court may review all the facts and circumstances.”

Better Markets said the lawsuit was probably the first of its kind.

JPMorgan declined to comment. In a statement, the Justice Department said it was “confident that the settlement reached with JPMorgan Chase complies with the law.”

While the Better Markets lawsuit faces an uncertain future, the case could muddy the image of an investigation that put Wall Street on high alert. The deal, lawyers and bankers say, laid the groundwork for a number of other approaching settlements with big banks.

For JPMorgan, the deal resolved an array of state and federal investigations. Under the terms of the settlement, JPMorgan will distribute the $13 billion to several states, the Justice Department and an alphabet soup of federal agencies, including the Federal Housing Finance Agency. Some money is also earmarked to help struggling homeowners.

Dennis Kelleher, the head of Better Markets, was an instant critic of the deal. When the settlement was completed in November, Mr. Kelleher questioned why the Justice Department declined to air its accusations in a lawsuit before settling with JPMorgan. A settlement reached after a lawsuit would have been subject to judicial approval.

Instead, the Justice Department published on its website an 11-page “statement of facts” outlining the bank’s wrongdoing. The actual settlement deal, a 16-page document detailing the terms of the settlement, is also public.

But Better Markets argues that those documents have some important omissions. The Justice Department, the lawsuit notes, “did not disclose the identity of a single JPMorgan Chase executive, officer or employee, no matter how involved in or responsible for the illegal conduct.”

Better Markets also highlighted some ambiguity about the breadth of the wrongdoing covered in the deal. One government document stated that the investigation spanned from 2005 and 2008, while another document refers to activity from 2005 to 2007.

“This contract was the product of negotiations conducted entirely in secret behind closed doors,” the lawsuit said. “No one other than those involved in those secret negotiations has any idea what JPMorgan Chase really did or got for its $13 billion because there was no judicial review or proceeding at all regarding this historic and unprecedented settlement.”