U.S. Senator Amy Klobuchar, D-Minn., called on the Obama administration Tuesday to review the effects of past airline mergers as it examines the proposed American Airlines-US Airways merger. A letter signed by Klobuchar and Sen. Mike Lee, R-Utah, calls for an assessment of how past mergers, including the Northwest Airlines-Delta merger, have affected prices, service, fees and mileage programs. "As you know, over the past few years, we have seen significant consolidation in the airline industry," the letter states. "Mergers between Delta and Northwest and United and Continental have reduced the number of legacy carriers from six to four. In addition, two of the largest low-cost carriers – Southwest and AirTran – merged in 2011." The letter was addressed to Attorney General Eric Holder and outgoing Secretary of Transportation Ray LaHood. Klobuchar, chairwoman of the Senate Judiciary Committee's Antitrust Subcommittee, said she will hold a hearing on the proposed merger on March 19. Lee is the ranking Republican on the panel.