The US Supreme Court Monday dealt a blow to mandatory minimum sentencing, ruling that any facts used to trigger a mandatory minimum sentence are "elements" of the crime and must be proven by a jury, not left to a judge. The 5-4 ruling came in Alleyne v. United States.

Until Monday's ruling, judges had been able to find certain facts that would trigger mandatory minimum sentences, such as quantities of drugs involved in an offense, based on a "preponderance of evidence" in post-conviction sentencing hearings. Now, those facts will have to established by juries in the course of the trial using the higher standard of proof "beyond a reasonable doubt."

The case is the latest in a line of cases that began with the groundbreaking 2000 Supreme Court decision in Apprendi v. New Jersey, which held that any fact that increases the range of punishments is an "element" of the crime and must be presented to a jury and proved beyond reasonable doubt.

Sentencing reform advocates were pleased by the ruling.

"Mandatory minimums for drug offenders will lessen, but it's difficult to say to what extent," said Marc Mauer, executive director of the Sentencing Project, which opposes mandatory minimum sentences. "It's also likely that this will have beneficial effects in reducing racial disparity, because so many mandatory minimums are imposed for drug offenses, and because African-Americans in particular are on the receiving end of those penalties."

"No defendant should have to face a mandatory minimum sentence because of facts that are not considered -- or worse, considered and rejected -- by a jury," said Mary Price, vice president and general counsel for Families Against Mandatory Minimums (FAMM), which submitted a friend of the court brief in the case. "As Justice Thomas noted in Monday's opinion, 'mandatory minimums heighten the loss of liberty.' Today, those who face mandatory minimums do so with the Constitution more firmly at their backs."

Drug offenders are those most likely to be hit with mandatory minimum sentences.