The U.S. Department of Transportation said on Tuesday it is looking into a complaint that Delta Airlines Inc used "unfair and deceptive practices" to block development of a second major airport near Atlanta.

The agency's action comes in response to a complaint filed by county commissioners and a private group trying to develop commercial air service at Silver Comet Field, a small airport about 40 miles from Atlanta. The DOT also received a letter opposing the plan from newly elected county commissioners due to take office in January.

The DOT action is not a formal investigation, as requested by airport proponents. The agency told Reuters it is "looking into the matter" after receiving the complaint in a letter to U.S. Transportation Secretary Anthony Foxx last week.

The developments, which have not been previously reported, are the latest in a long-running dispute over efforts to create an alternative to Hartsfield-Jackson Atlanta International Airport.

Atlanta, the No. 9 U.S. metropolitan area, is the only one of the top 10 without at least one secondary airport, according to Census Bureau and Federal Aviation Administration data.

Last October, the Paulding County Board of Commissioners said it planned to bring commercial air service to Silver Comet Field, in a county northwest of the city.

In 2012 it signed a deal to develop airport infrastructure with Propeller Investments, a New York-based private equity firm. Allegiant Travel Co , a low-cost carrier, has said it intends to serve Silver Comet Field.

Delta has long opposed a second commercial airport in the Atlanta area. Chief Executive Richard Anderson has said it would divide investment and "ultimately be an economic and community failure." Atlanta's mayor has also opposed the development.

In the letter to Foxx, Paulding County Commission Chairman David Austin and Robert Aaronson, Chairman of Propeller Airports and a former senior official at FAA and the Port Authority of New York and New Jersey, said Delta wants to preserve its dominance in Atlanta.

They said Delta is "interfering" with efforts to develop low-cost air service around Atlanta, even though the Silver Comet plan has been "endorsed by Paulding County, the State of Georgia and the FAA."

Delta's actions "are harming consumers and stifling economic development in the region," they said in the letter.

Paulding County and Propeller officials said, that since the agreement was signed last October, a series of incidents has stalled the project. These include lawsuits challenging the development brought be country residents through an Atlanta law firm did not charge them legal fees.

A detailed chronicle of events filed with the DOT complaint letter also notes break-ins at the homes of a state representative whose district includes Paulding County, and the county commission chairman, who support the project.

In a separate letter last week, three newly elected members of the five-member Paulding County Board of Commissioners asked Foxx and FAA Administrator Michael Huerta to halt commercialization of the airport.

The members, who take office in January, said the current county board "conspired behind closed doors" to start air service at Silver Comet, despite objection by residents who voted in three "anti-commercialization" candidates last May.

Delta said its objections are shared by local property owners, environmental groups and labor unions. The airline declined to comment on the legal backing of local residents who oppose the airport expansion.

"The proposal is a waste of taxpayer dollars, violates the city's restrictions on the land, and would siphon off increasingly scarce federal funding that's more needed at Hartsfield-Jackson Atlanta International Airport," Delta spokesman Trebor Banstetter said.

"Metro Atlanta is best served by a single, strong airport at Hartsfield-Jackson," which has enough room for new airlines to being service, he added. Delta has agreed to work with the airport to accommodate new entrants if necessary."