A nasty dispute over a key piece of land near the $975 million Vikings stadium site got stickier Friday when the public board overseeing the project charged the property owner with "grossly" overstating its value.

In a counterclaim to a lawsuit filed last week in Hennepin County District Court, the Minnesota Sports Facilities Authority alleged that Minneapolis Venture LLC, which owns the land under the Downtown East light-rail stop, has greatly inflated the property's development potential and value — nearly double the authority's appraised value —in order to squeeze millions of dollars from the board at a time when a tentative construction start on the stadium is only weeks away.

It also contends that Minneapolis Venture cannot terminate a 2003 agreement with the Metropolitan Sports Facilities Commission, the authority's predecessor, over using a portion of the block as a public plaza before NFL games or other sporting events. That agreement expires Oct. 31.

Michele Kelm-Helgen, the authority chairwoman, Friday called the threat to end the plaza deal "a negotiating tactic" by Minneapolis Venture to gain leverage in the valuation dispute.

"They know it's an important piece of property to their project," she said. "And to me, they are grossly overstating what the value is. From the public's perspective, there's no way we could ever agree to that."

The authority's response comes as it races to complete a thorough financial and legal background check into team owners Mark and Zygi Wilf in time to meet a tentative, Nov. 7 stadium groundbreaking date.

The "due-diligence" review was prompted by a New Jersey judge's ruling earlier this month that the owners had defrauded partners in a real estate deal in that state. The judge, who was highly critical of the way the Wilfs do business, is expected to rule on damages, which could cost the owners tens of millions of dollars or more, sometime in coming weeks.

Minneapolis Venture filed a suit against the stadium authority earlier this month claiming that negotiations over the property have been "perplexing and unproductive."

It asked a judge to rule on whether the above-ground plaza and its coveted underground parking garage are part of the massive stadium's reach, stating that at times, it was unclear whether the authority even wanted to purchase the land although "it was communicating to the public that Downtown East was within its control."

Jon Austin, a spokesman for Minneapolis Venture, said Friday in response to the authority's counterclaim: "In the space of a few short pages, they manage to stake a nonexistent claim to our property by implying that even though we own the land we cannot use it, to falsely assert that we have some unwritten obligation to agree to any terms they think appropriate and to demand — without any basis in fact, contractual language or law —that we dedicate our land to be used by the Vikings and the MSFA without compensation."