U.S. Attorney Joseph Hogsett is resigning as the top federal prosecutor for central and southern Indiana after nearly four years in the position.The decision announced Monday by the former Indiana secretary of state follows months of speculation that he will seek the Democratic nomination for Indianapolis mayor next year.Hogsett's office says his resignation will be effective July 31. His resignation letter to Attorney General Eric Holder doesn't give a reason for the decision.Hogsett had said earlier this year he intended to remain U.S. attorney through the end of his term in 2016.Republican Indianapolis Mayor Greg Ballard hasn't yet said whether he'll seek a third term in the 2015 election.