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.