Middlesex District Attorney Gerard T. Leone Jr., who has prosecuted some of the biggest cases in the state, including that of attempted shoe bomber ­Richard Reid and nanny Louise Woodward, surprised the legal community Thursday night when he announced that he would not seek reelection to a third term.

The Democrat, who made his declaration during an awards ceremony for his staff, also said he intends to leave government service when he departs from ­office, according to a transcript of his remarks released by his spokeswoman.