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Washington (CNN) -- Federal and state governments should move to a single lethal drug for executions instead of complex cocktails that can be botched, a group of criminal justice experts said in a new review of the death penalty in the United States.
The experts also recommended that governments should let the public know more about their processes in carrying out the death penalty.
The report comes as the Justice Department conducts a review of federal and state death penalty procedures, including the drugs used, following a botched execution in Oklahoma last week.
The Constitution Project published the report called "Irreversible Error," the work of bipartisan group of experts, including former elected leaders who oversaw executions, and anti-death penalty activists.
The study was already in the works before the failed execution in Oklahoma.
The group doesn't take a position on whether the death penalty should be used, but rather suggests ways to improve the system, the Constitution Project says, to ensure it is being carried out fairly.
"Without substantial revisions -- not only to lethal injection, but across the board -- the administration of capital punishment in America is unjust, disproportionate and very likely unconstitutional," former Virginia Republican Attorney General Mark Earley, a member of the committee that produced the report, said in a statement.
Former Texas Gov. Mark White, a Democrat and death penalty supporter who co-chaired the committee, said in a recent opinion piece in Politico that he regrets airing an election ad in which he boasted of overseeing the executions of 19 people during his time in office.
Among its 39 findings, the report warns that executing people convicted of felony murder violates the Eighth Amendment to the Constitution, which bans cruel, unusual and excessive punishment for crimes.
In some states, someone could be convicted of felony murder without killing or even intending or attempting to kill, the report says.
"Including accidental and non-intentional murderers among the death-eligible creates perverse outcomes — those least "deserving" of a death sentence can be sentenced to die while premeditated and intentional murderers may avoid capital punishment," the report says.
The report suggested improving clemency procedures and better access to the courts for defendants on death row.
The report also takes on the issue of executing people suffering from mental retardation, which the Supreme Court banned in 2002. Some states, the report says, have overly stringent standards for people to be declared mentally handicapped.
The move to a single lethal drug, the report says, would make sure the government is using "the most scientifically reliable methods to minimize pain and suffering during an execution."
States also should make public what drug is used and allow the public to have input on the process. Currently, some states do not identify the drugs they use for lethal injections. Some manufacturers refuse to allow their drugs for such use.