A federal judge’s line of questioning yesterday made it appear he was unconvinced when defenders of the Atlantic Yards project argued that the public benefits of the proposed development protect it from a lawsuit.

During lengthy oral arguments, a magistrate judge in U.S. District Court in Brooklyn, Robert Levy, asked probing questions to lawyers on both sides about where he should draw the line between legal and illegal uses of eminent domain.

The Atlantic Yards project, which calls for a basketball arena, nearly 7,000 units of housing, and office space, would cover 22 acres in Prospect Heights, Brooklyn, some of which is currently in private hands. The proposal is being challenged in a lawsuit by three homeowners, several renters, and a business owner. Their properties face condemnation and seizure under the plan.

They are suing the project developer, Forest City Ratner, as well as Mayor Bloomberg and Governor Pataki, who support the project. The 10 named plaintiffs say any public benefits are incidental to the project’s ultimate goal of enriching Forest City Ratner.

The central issue during yesterday’s hearing was whether Judge Levy would be overstepping his role if he began to weigh the public benefits that the project would deliver against the private benefits that would accrue to Forest City Ratner. The defendants are asking Judge Levy to immediately dismiss the lawsuit on the grounds that the public benefits of the development are obvious.

A lawyer for Forest City Ratner, Jeffrey Braun, told Judge Levy that his role as a reviewer of the plan should be extremely limited. All the judge needed to do before recommending the dismissal of the lawsuit was to confirm that the proposed public uses were legitimate, Mr. Braun, said.

“What I’m hearing is that so long as a defendant can articulate a public use,” Judge Levy asked, eminent domain is “per se constitutional?”

“Your honor, I would say yes,” Mr. Braun said, adding that “the role of the court really disappears” beyond conducting that initial review.

This answer did not seem to satisfy Judge Levy. Soon thereafter, he put forward a hypothetical: What if developers wanted to seize 22 acres of private property and build luxury housing on 21 acres of it and place a school on the remaining acre?

Under Mr. Braun’s argument, Judge Levy noted, “Any condemnation would pass the test so long as a school was put in, regardless of the detriments.”

“When can a court say,” Judge Levy asked, “as a matter of law, we’re not going to dismiss the case because an allegation of public use isn’t sufficient in this particular project?”

In response to another hypothetical posed by Judge Levy — this one positing corruption between a developer and the government — Mr. Braun backed down from his position.

“This is where my dilemma is,” Judge Levy said. “How do I know where to draw the line?”

Indeed, by his questions, Judge Levy appeared eager to weigh the public benefits against the private motives at work in the Atlantic Yards proposal. But the judge also noted that such an accounting would not necessarily lead to any easy determination as to whether the use of eminent domain is constitutional.

While defenders of the proposed plan were touting the public benefits, Judge Levy noted that the plaintiffs were more interested in pointing out the private gain that Forest City Ratner anticipated.

“What if we have both?” Judge Levy asked. “Let’s say there is an undeniable public use and let’s say at the same time there is an undeniable private benefit.”

In that case, a lawyer for the plaintiffs, Matthew Brinckerhoff, said he would urge Judge Levy to decide what is the primary goal of the project.

“The standard would be what was the motivating factor in taking somebody’s property,” Mr. Brinckerhoff said. If it is for private gain, then eminent domain “has to be stopped, it is unconstitutional.”

Judge Levy is expected to issue a recommendation on whether to dismiss the case to the presiding judge overseeing the case, Nicholas Garaufis.