The 5-2 decision by the Connecticut Supreme Court follows a 2012 law that banned the imposition of death sentences on newly committed crimes but allowed the state to proceed with the executions of inmates who had already been sentenced to die. The court in August ruled that exception was unconstitutional, amounting to a cruel and unusual punishment.
State prosecutors challenged the ruling, arguing that the court had overstepped its authority by rejecting an exception allowed by the legislature.
Related: Controversial death penalty cases:
Controversial Death Penalty Cases
Connecticut's top court upholds ban on death penalty in state
Dave Atwood, left, and Sophia Malik, right, both of Houston, hold photos of Napoleon Beazley as they protest his execution Tuesday, May 28, 2002, in Huntsville, Texas. Beazley, 25, was executed by lethal injection for the 1994 carjacking murder of 63-year-old John E. Luttig of Tyler, Texas. It was the 14th execution this year in Texas. (AP Photo/Brett Coomer)
Rena, left, and Ireland Beazley hold a photo of their son Napoleon Beazley at their home in Grapeland, Texas, Friday, May 31, 2002. Napoleon Beazley's death sentence for killing the father of a federal judge during a 1994 carjacking at age 17 stirred national debate over capital punishment for youths. (AP Photo/Donna McWilliam)
Rena Beazley, left, and her husband, Ireland, from Grapeland, Texas, are shown in the audience during a news conference Thursday, May 23, 2002, in Austin, Texas. The two, parents of Texas death row inmate Napoleon Beazley, and clergy pleaded for his sentence to be commuted to life in prison. He is scheduled to be executed by lethal injection Tuesday. (AP Photo/Harry Cabluck)
Mugshot of Cameron Todd Willingham
(Photo credit: Texas Department of Criminal Justice)
Judy Cavnar, of Ardmore, Okla., a cousin of executed Texas prison inmate Cameron Todd Willingham, displays a picture of him during a news conference Tuesday, May 2, 2006, in Austin, Texas. The case of a Willingham, who maintained his innocence until the end but was executed after he was convicted of an arson murder, is going before a new state commission required to look into allegations of forensic misconduct. (AP Photo/Harry Cabluck)
Eugenia Willingham of Ardmore, Okla., right, wipes a tear as she speaks during a news conference Tuesday, May 2, 2006, in Austin, Texas. Willingham and other relatives of Cameron Todd Willingham recounted the final moments of Willingham's life and their unsuccessful attempts to block his execution. The New York-based Innocence Project submitted the case to the Texas Forensic Science Commission on Tuesday and also asked the panel to review arson convictions statewide. In the background, from left are Willingham's cousins, Pat Cox, and Judy Cavnar. Mrs. Willingham is his stepmother. (AP Photo/Harry Cabluck)
Death row inmate Troy Davis appears in this undated file photo provided by the Georgia Department of Corrections. (Georgia Department of Corrections/MCT via Getty Images)
Demonstrators gather in front of the White House in Washington as they hold a vigil before the scheduled execution of death row inmate Troy Davis, Wednesday, Sept. 21, 2011. Davis is facing lethal injection for killing an off-duty Georgia policeman in Savannah, a crime he and others have insisted for years that he did not commit. (AP Photo/Charles Dharapak)
A man chants during a vigil for Georgia death row inmate Troy Davis In Jackson, Ga., Wednesday, Sept. 21, 2011. Davis is scheduled to die Wednesday for the killing off-duty Savannah officer Mark MacPhail. (AP Photo/John Bazemore)
Anne MacPhail pauses for a moment after learning at 10:55 p.m., on September 21, 2011, that the U.S. Supreme Court had denied a stay of execution for Troy Davis, who was convicted in the 1989 murder of her son Mark MacPhail. Davis was executed shortly after in Jackson, Georgia. (Robin Trimarchi/Columbus Ledger-Enquirer/MCT via Getty Images)
Mugshot of Kelly Renee Gissendaner
(Photo credit: Georgia Department of Corrections)
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The five justices said in a brief, two-paragraph opinion, that there was no reason to reverse their earlier decision. In a supporting opinion, Justice Richard Palmer cited concerns that the capital sentences are more likely to be handed down against black defendants.
"Any sentencing scheme that allows prosecutors not to seek and jurors not to impose the death penalty for any reason 'necessarily opens the door' to caprice and bias of various sorts, racial or otherwise," Palmer wrote.
Thursday's decision came in response an appeal filed by lawyers for former drug dealer Russell Peeler Jr., who was sentenced to death for ordering the 1999 killing of an 8-year-old boy and his mother because he believed the child had planned to testify against him in another case.
The court's initial decision that the death penalty was unconstitutional came in the case of Eduardo Santiago, who was convicted of the 2000 murder of the romantic rival of an associate in exchange for a snowblower.
In a dissenting opinion, Justice Peter Zarella argued that the court's decisions in both cases were wrong.
"I cannot fathom how Chief Justice (Chase) Rogers and Justice (Richard) Robinson believe they respect the rule of law by supporting a decision that is completely devoid of any legal basis," Zarella wrote.
Nineteen U.S. states including Connecticut have banned the death penalty, while 31 still have it on the books.
Connecticut has not executed a prisoner since 2005, when serial killer Michael Ross, who admitted killing eight women in the 1980s, was put to death by lethal injection.
The state's Democratic Governor, Dannel Malloy, said the court's decision reflected the will of the people.
"Those currently serving on death row will serve the rest of their life in prison with no possibility of ever obtaining freedom," Malloy said in a statement. (Reporting by Scott Malone in Boston; Editing by Matthew Lewis)