Tyngsborough Police Chief William Mulligan, who has been on medical leave since December 2012, has announced that he will retire when his contract expires on June 30. Deputy Chief Richard Howe, who has been acting chief since Mulligan’s leave began, will take over the position temporarily until a permanent chief is appointed. Mulligan has been in law enforcement for more than 30 years, having started his career in Merrimack, N.H. He became police chief in that community before joining the Tyngsborough department in 2007. At a recent Board of Selectmen meeting, selectmen expressed gratitude for the work Mulligan has done, and wished him well.