The state’s highest court on Wednesday narrowed the scope of the state’s longtime felony murder law, ruling that defendants in fatal crimes can no longer be convicted of first-degree murder unless it is proven that they set out to kill or knew their actions would likely turn fatal.
The ruling affects accomplices who drove the getaway car or otherwise participated in a crime — but weren’t the actual “triggerman.’’ It also will affect a person who committed a felony — such as a rape or a robbery — and a victim died sometime later.
The Supreme Judicial Court ruling raises the burden of proof in such cases, making it more difficult for prosecutors to secure first-degree murder convictions, legal specialists said. It is not retroactive.
“A prosecutor is going to have satisfy a jury that there was malice,” said Rosanna Cavallaro, a professor at Suffolk University Law School. “They must convince a jury that defendants intended to kill, and not just that they had intended a robbery and had bad luck.”
Until now, “the intent to commit the felony is sufficient alone to establish malice,” Chief Justice Ralph D. Gants wrote in the ruling.
The court established the new standard in the case of a Lowell man who was convicted of first-degree murder in 2013 after a jury found he supplied a pistol and hooded sweatshirts to intruders who fatally shot two brothers during a botched robbery in 2009.
The defendant, Timothy Brown, 29, wasn’t present when the killings occurred. The justices unanimously agreed to set aside his first-degree murder conviction and enter a verdict of second-degree murder, making him eligible for parole.
Under Massachusetts law, defendants convicted of first-degree murder are automatically sentenced to life in prison without the possibility of parole.
“I believe that, in the future, a defendant should not be convicted of murder without proof of one of the three prongs of malice: that he or she intended to kill or to cause grievous bodily harm, or intended to do an act which, in the circumstances known to the defendant, a reasonable person would have known created a plain and strong likelihood that death would result,” Gants wrote.
The decision will apply to felony murder cases going forward. Three justices — Frank M. Gaziano, David Lowy, and Elspeth Cypher — opposed the change.
“In my view, the abrogation of common-law felony-murder . . . at the expense of innocent victims of violent crime, is not necessary,’’ Gaziano wrote. “There will be instances where morally culpable individuals will not be held responsible for the death of a rape victim, gasoline station attendant, or convenience store clerk.’’
The felony murder rule is part of common law and is used in other states. However, some states — Michigan, Hawaii, and Kentucky — have revised their laws.
Plymouth District Attorney Timothy J. Cruz criticized the decision.
“As pointed out by Justice Gaziano, the decision unfairly diminishes the seriousness of violent felonies that result in the deaths of innocent victims,” he said in a statement.
A spokeswoman for Middlesex District Attorney Marian T. Ryan, whose office prosecuted Brown, said prosecutors are reviewing the decision.
Gants said the change was needed to correct constitutional errors dating back to 1863, when a man was shot to death during a Civil War antidraft riot, and it was not clear if the shooter was a soldier or a rioter.
But the defendant was tried for the murder on the grounds that he bore criminal responsibility, even if he was not the shooter. That principle has since been applied ever since in the Massachusetts courts.
No longer, Gants wrote.
“Criminal liability for murder in the first or second degree will be predicated on proof that the defendant acted with malice or shared the intent of a joint venturer who acted with malice,’’ Gants wrote in his majority opinion.
Gants disputed Gaziano’s examples, saying the rapist could still be tried and convicted of murder under the new rules. The convenience store thief, if there were no evidence that the shooting was deliberate, would face a reduced charge of involuntary manslaughter, Gants said.
“We must revise that common law so that it accords with those fundamental principles and yields verdicts that are just and fair in light of the defendant’s criminal conduct,” Gants said.
Martin W. Healy, chief legal counsel of the Massachusetts Bar Association, said the ruling “upends centuries-old principles’’ and that many people are serving life sentences under the rule narrowed by the court.
“What you are seeing are judges requiring a higher standard of evidence in terms of convicting someone of felony murder,’’ Healy said. “It really ties prosecutors’ hands.”
Boston attorney David Nathanson, who is handling appeals in three felony murder cases, said the ruling fixes “the ultimate technicality.”
“You’re sentencing someone to die in prison based on their intent to commit some other felony which could be as mild as unarmed robbery,” he said. “The government is supposed to prove all essential elements of an offense, and one of them is that you intended to commit the offense.”
David Mirsky, a lawyer who defended Brown, said he hoped the SJC would acquit Brown of the killings.
“I maintain my client is not guilty,” Mirsky said. “People should not be punished for matters where they lack criminal intent.”
Gaziano argued the court should have left the current rule intact and instead made careful use of another law that allows the SJC to reduce murder verdicts when they conclude justice was not done.
Under the revised standard, certain prosecutions would no longer be feasible, Gaziano wrote.
He cited the example of a man who raped an elderly woman who survived the attack but died three weeks later from an infected wound. The man was convicted of first degree murder in 1998.
“This approach, which is predicated on an extremely narrow view of moral culpability (or blameworthiness), diminishes the seriousness of violent felonies that result in the deaths of innocent victims,” he wrote.Martin Finucane of the Globe staff contributed.