The Justice Department has requested information from airlines as part of an investigation into whether airlines are colluding to grow at a slower pace as part of an effort to keep airfares high. A DOJ spokeswoman confirmed Wednesday that the department was investigating potential “unlawful coordination” among some airlines. She declined , however, to comment further, or to identify which airlines are being investigated. As a result of a series of mergers starting in 2008, American Airlines, Delta Air Lines, Southwest Airlines and United Airlines now control more than 80 percent of the seats in US skies. The airlines say they are not doing anything wrong, and their revenue numbers prove it.