“We have not reached an agreement with Dean Foods for its assets, so it may be premature, but the Justice Department has a job to do,” Ms. Massey said. “If we reach a deal, we would fully cooperate with D.O.J. officials as we have with past transactions.”

The Justice Department appears to have been monitoring the situation since the early stages of the Dean Foods bankruptcy proceeding, when department lawyers filed notices of appearance in the case, effectively declaring that the government had an interest in the outcome. A spokeswoman for Dean Foods declined to comment.

On Dec. 20, an antitrust official at the Justice Department, Karl Knutsen, called Mr. Zakarian and Joel G. Beckman, another lawyer working on the Dairy Farmers of America lawsuit in Vermont, asking them to waive a confidentiality provision to allow investigators to read depositions and other documents in the case.

The Vermont suit alleges that Dairy Farmers of America has engaged in a wide range of anticompetitive practices over the years, striking deals with other co-ops not to poach one another’s members and sharing milk-pricing information to suppress payments to farmers.

Dairy Farmers of America has denied those allegations. But in September, the judge overseeing the case allowed it to move to a trial, writing in her decision that the plaintiffs had presented evidence from which a “rational jury could conclude that the D.F.A. management favored growth of its commercial operations and empire building over the interests of its farmer-members.”

The Justice Department’s apparent interest in the Vermont case might indicate that government lawyers are weighing a broader investigation into Dairy Farmers of America’s practices that would go beyond the possible merger with Dean Foods, said Peter Carstensen, a former antitrust lawyer at the Justice Department.

“They’re really focusing in on the impact on farmers and how this kind of transaction could greatly increase the capacity of D.F.A. to engage in anticompetitive practices,” Mr. Carstensen said. And that line of inquiry could lead government investigators to “take a much more critical look at the way D.F.A. is using its existing market positions and think about whether there are workable remedies for that conduct.”