Adams County launched a grenade Friday in an ongoing dispute with Denver over how Denver International Airport should develop, demanding the land Denver annexed 25 years ago for the airport be returned.

The county sent two letters to Denver Mayor Michael Hancock — one that proposes that Denver de-annex the 54 square miles that was annexed in 1988 after years of negotiations and approval by voters.

The other letter was a “notice of default” that says Denver’s plans to create an “airport city” with non-aviation commercial development on DIA is a violation of the intergovernmental agreement and would result in de-annexation.

The default letter is a precursor before Adams can file a lawsuit.

Adams commissioners issued a statement saying citizens expect their governments to be trustworthy.

“Adams County will honor and, as necessary, enforce its voter-approved agreement allowing annexation of 54 square miles for the development of DIA by Denver,” the statement said. “The promises made by Denver to Adams County citizens must be kept.”

Denver Mayor Michael Hancock, preparing to lead a delegation to Japan next week to inaugurate a much sought-after nonstop flight to Asia from Denver, called Adams County’s letters a great disappointment and the proposal “a joke.”

“Their irresistible temptation to slap Denver really overrides their ability to think broadly and cooperatively about the real economic gem that we have,” he said. “They simply can’t let go of the 25-year-old history and dislike for Denver to work cooperatively with us enough to benefit the people we are elected to serve.”

Long-standing friction between the two counties was renewed when in 2012 Denver announced plans to build an “airport city” on DIA property and work with surrounding communities to build out an “aerotropolis” outside of the airport.

The vision is a cornerstone of Hancock’s administration, which notes that the airport, halfway through its build-out, employs 30,000 and has an economic impact of more than a $20 billion.

Adams officials countered that Denver was bound to the terms of the IGA that specified exactly the types of development that could occur on DIA, a deal inked to ensure that Denver doesn’t reap all of the economic benefit from the airport in exchange for the annexation of the land.

Last month, Adams officials asked Hancock to provide specific and tangible benefits that the airport city idea would provide Adams County.

Hancock responded last week with a proposal of a special district that would collect taxes on the new businesses on and around DIA with the majority of revenue used for public infrastructure projects in an off-airport service area in Adams County, Aurora, Commerce City and Denver.

Aurora Mayor Steve Hogan told The Denver Post on Wednesday that he was against the proposal, saying it wasn’t a good deal for Adams County and its communities.

In their letter to Hancock, Adams County commissioners said:

“Denver’s proposed plan for ‘sharing’ revenue does not even begin to make Adams County and its cities whole in exchange for their potential surrender of the benefits afforded by the IGA.”

Instead, Adams officials propose that Denver de-annex the entire DIA property, returning it to Adams County.

“Denver would remain the landowner and airport operator and the income derived from the developed land could be used to defray airport costs, lower gate fees, increase DIA’s competitive advantage over other airports and hopefully attract more airlines and routes,” said the letters signed by all three commissioners. Adams County would collect the tax revenue, the letter proposes.

“This is a true regional solution, as all entities and their citizens benefit from the growth,” it says, adding that the proposal would create an agreement much like the setup at Dallas Forth Worth airport.

“Dallas and Fort Worth jointly own and operate the airport, but the airport is not located within their municipal territories,” the letter says.

“Our proposal would allow Denver to proceed unimpeded with the mutually beneficial regional development that has been proposed while maintaining the purpose of the 1988 IGA by ensuring that such development takes place within Adams County.”

Hancock said the proposal is ludicrous.

“They are saying, ‘Hey, I tell you what, if you want to develop the airport, give us back the land,’ which we spent millions of dollars for,” Hancock said. ” ‘Go ahead and develop the airport and we will just tax you to death.’ I don’t know if they thought they were being funny, but this is a serious game.”

“At some point we have to realize we are negotiating with people who don’t want to negotiate in good faith,” he said. “They are not at the table to have genuine conversation.”

Jeremy P. Meyer: 303-954-1367, jpmeyer@denverpost.com or twitter.com/jpmeyerdpost