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California prosecutors have said they're weighing capital punishment for the man accused of strangling an 11-year-old girl in 1973 — but legal experts insisted Thursday that's not possible.

James Alan Neal, 72, was charged Wednesday with murder for the slaying of elementary school student Linda O’Keefe in Newport Beach and Orange County District Attorney Todd Spitzer said at news conference that seeking the death penalty is an option.

Linda O'Keefe was last seen alive in Corona Del Mar, California, in July 1973. Newport Beach Police Department

But when O'Keefe vanished while walking home from summer school on July 6, 1973, and was found dead a day later, there was no capital punishment on the books in California.

"I'm certain that you can’t impose a higher penalty for a crime than that which existed at the time of the crime," veteran Los Angeles criminal defense lawyer Steve Cron said. "That’s what the prohibition against ex post facto laws is all about."

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California's Supreme Court, in February 1972, ruled that capital punishment amounted to cruel and unusual punishment. State lawmakers re-enacted the death penalty statute in 1977 before voters reaffirmed capital punishment at the ballot box in 1978.

California resumed executions in 1992. The state now executes prisoners by lethal injection.

"If the crime had been committed five years later, I have no doubt DA Spitzer would seek the death penalty," said Leif Dautch, vice chairman of the Criminal Law Executive Committee of the California Lawyers Association.

"Under settled law from the California Supreme Court, District Attorney Spitzer is barred from seeking the death penalty for a crime that was committed at a time when California did not have a valid death penalty." he said.

Still, at the press conference Wednesday announcing Neal's arrest, Spitzer insisted the death penalty is possible.

"And death may be a consideration in this case. As a district attorney, we have a process in place by which we would consider death and I will follow that process," he said. "I will make a decision about whether death is appropriate in this case."

Neal, arrested Tuesday morning in Colorado, made a brief appearance in an El Paso County courthouse Thursday. He is being held without bond and was ordered to return to court Feb. 28 to determine if he will be extradited to California.

A poster of James Alan Neal during the Newport Beach police and Orange County prosecutors news conference on Feb. 20, 2019. KNBC

One of O'Keefe's two surviving sisters said she's long ago forgiven the suspected killer.

“This is the additional closure for me that’s there’s a person they’ve arrested, but I forgave the individual a long time ago because unforgiving to me is like taking poison and wanting the other person to die,” Cindy Borgeson told NBC Tucson affiliate KVOA on Wednesday.

“He’s not a monster to me. He’s a man who made a bad decision that affected my family, and I believe in justice and that’s not up to me to decide what that looks like,” she said.