Three federal judges weighing Virginia’s challenge to the federal health care law have asked lawyers for the commonwealth and for the Obama administration to file additional briefs with the court.

In an order filed Monday, Judges Diana Gribbon Motz, Andre M. Davis and James A. Wynn requested that lawyers for each side file a brief of no more than 10 pages by May 31 answering specific questions regarding the anti-injunction act, which deals with the power of Congress to levy taxes.

They lodged an identical request with lawyers for Liberty University, which is also challenging the law.

Virginia’s suit, filed by Attorney General Ken Cuccinelli, argues that Congress exceeded its authority under the Constitution with a provision of the federal health-care overhaul that requires all individuals to obtain health insurance by 2014 or pay a fine.

One of the federal government’s arguments has been that the fine is essentially a tax and Virginia’s case should be thrown out because Congress has the power to levy taxes.

That argument was rejected at the federal district court level by Judge Henry E. Hudson. But the questions asked by judges in their request for supplemental briefing suggests they are looking closely at the issue.

University of Richmond School of Law Professor Carl Tobias said it is “unusual but not unprecedented” for federal judges to request additional written briefs after hearing oral arguments, as they did in the health-care cases this month.

It might suggest, he said, that as they have been writing their opinion, they got hung up on an issue they felt had not been fully discussed earlier in the case.

He characterized the three questions asked by the judges as “very narrow” about a rather obscure area of law. And he said that, given the short turnaround provided to the lawyers, it might be that the request will not delay the publication of an opinion, expected in June or July.