A tentative deal to swap the massive former American Motors headquarters in Detroit in exchange for 11 acres of city-owned land needed to build a new criminal justice complex near I-75 has been reached— bringing the $520.3-million offer from billionaire developer Dan Gilbert's Rock Ventures, one step closer to fruition.

The agreement between Wayne County and the City of Detroit comes after officials told the Free Press last week that negotiations were ongoing and both sides were optimistic an agreement would be reached soon.

The deal also pushes forward an effort by a group led by Gilbert and Pistons owner Tom Gores to build a 23,000-seat stadium on the unfinished Gratiot jail site to host a Major League Soccer team. The investment has been estimated at $1 billion. The application for an expansion team filed in January listed only the jail site as a location.

Gilbert has previously said that construction would begin March 6, 2018, and be completed Nov. 5, 2020.

The acquisition of the city-owned land was crucial in order to move Rock Ventures’ proposal forward.

The proposal initially sought 13 acres of the 22-acre site, which is bounded by the I-75 Service Drive and E. Warren Ave., and includes Detroit Department of Transportation administrative offices.

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But officials said today the deal is now for just slightly over 11 acres of land behind DDOT's bus terminal, ensuring the transit's operations won't be impacted.

The deal would also enable the city to acquire the 1.4 million-square-foot complex of the former American Motors Corp. headquarters on Plymouth Road on Detroit's west side, which is currently owned by the Wayne County Land Bank.

“The city is swapping land we don’t need to the county to enable the county to build a criminal justice center,” Mayor Mike Duggan said in a statement. “In exchange, the county is swapping a vacant parcel in west Detroit that the city wants to take on as a redevelopment project to improve the neighborhood. This is a win-win.”

Duggan said the city plans to market the old headquarters for re-development but the subsequent purchase agreement must first clear hurdles of approval by the Detroit City Council, Wayne County Commission and the Wayne County Land Bank Board.

But according to the agreement, Detroit has the option to forgo swapping the land and instead opt to receive the fair market value of the city property from Wayne County, which is estimated to be $757,000. The city has until Nov. 1, 2018 to make that selection.

“This agreement gets us one step closer to a deal with Rock Ventures. Acquiring this land was one of the significant hurdles to Rock’s proposed criminal justice complex,” Wayne County Executive Warren Evans said in a statement. “We worked out a tentative deal that makes a lot of sense for the city and county, and we look forward to finalizing the agreement.”

The old headquarters at Plymouth Road is well-known across metro Detroit for its distinctive brick tower, which dates to 1927 and once housed the manufacturer of Kelvinator refrigerators.

But it's mainly known for its strong ties to the region's auto industry.

The Nash-Kelvinator Corp. merged in 1954 with the Hudson Motor Car Co. to form American Motors, which used the complex as its headquarters and made Jeeps there.

When Chrysler acquired American Motors in 1987, the property became the engineering center for all Chrysler and Jeep trucks and SUVs.

Chrysler kept employees in the building as late as 2009 before shifting them to its Auburn Hills headquarters. The Free Press previously reported the building and nearby parking lots were sold for $2 million in 2010 after Chrysler's bankruptcy to a corporate entity called Plymouth Road One LLC, which was listed under the name of Terry Williams, a Detroit scrapper hauler.

But one year later, Williams, pleaded guilty and was sentenced to 2 years, 3 months in prison for violating the Clean Air Act for disturbing materials containing asbestos.

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The county foreclosed on the property because of the more than $2 million in unpaid taxes owed by Plymouth Road One and two years ago, a Commerce Township man made a winning $500 bid for the property but the deal fell through. The property is currently owned by the Wayne County Land Bank.

And now with a tentative agreement in place, the county and city are working toward reaching a final agreement on the land swap.

Under the agreement, both the city and county have the ability to conduct a due diligence investigation of properties, along with environmental tests. If the city isn't satisfied with the results, it has the right to receive the fair market value for the property.

But unlike the city, the county has the option to terminate the agreement altogether, if not satisfied. The agreement also requires that both Evans and Duggan agree to "support and not object to any tax incentive or financial incentives" proposed for the respective properties they relinquished in the deal.

But if the county doesn't reach a development and purchase agreement with Rock Ventures and win the necessary approvals by Feb. 1, 2018, the land swap agreement could be terminated.

Two months ago, Wayne County announced it was moving forward with Gilbert's proposal which would also replace Wayne County's unfinished jail with a $1-billion commercial development.

Despite the agreement, an offer from Walsh Construction to complete the unfinished jail on Gratiot remains on the table, if the deal with Rock Ventures can't be worked out.

But officials have said the Walsh proposal puts the county at risk for cost overruns, unlike the Rock bid.

Walsh’s proposal includes two jail options at Gratiot: one with nearly 1,608 beds at $269 million and another with 2,200 beds costing $317.6 million.

The county has said it needs at least 2,000 beds.

Ground was broken for the proposed jail, near I-375, under the leadership of then-Wayne County Executive Robert Ficano in September 2011. Since then it has cost county taxpayers hundreds of millions of dollars. Work has been halted at the site since June 2013 as estimates showed cost overruns pushing the project $91 million over the $300-million budget. The idled site is costing taxpayers about $1.2 million a month in bond payments, storage and security.

But last week, some Wayne County commissioners raised concerns about how the negotiations for the proposal are moving forward, claiming they've been left in the dark.

County officials have rebuffed those claims, saying they've been forthright but the negotiations are complex in nature. The county has until Dec. 1. to have a definitive agreement for the commission to consider.

Contact Katrease Stafford: kstafford@freepress.com or 313-223-4759.

Reporter J.C. Reindl contributed to this report.