LANSING, Mich. (AP) — The constitutional rights of more than 1,000 inmates serving life sentences in Michigan prisons have been violated ever since parole policies were toughened in the 1990s, a federal judge has ruled.

U.S. District Judge Marianne Battani said the cumulative effect of the parole changes violates the Constitution's ban on laws being applied retroactively.

She released her decision this week but has yet to decide what her ruling means for 1,000 to 1,200 Michigan prisoners sentenced before 1992 to life in prison with the possibility of parole.

Since the early 1990s, the Michigan Parole Board has been less willing to release prisoners sentenced to life with the possibility of parole, adopting a "life means life" policy.

Some Michigan judges have said they never intended that some criminals remain in prison for life. Before the parole board stiffened its policy, those serving a life sentence with the possibility of parole could be released sooner than those give a fixed sentence such as 25 years.