LOS ANGELES (CBSLA.com)  — Authorities have arrested an Arcadia man they say is responsible for killing a toddler in Louisville, Ky., in 1971.

Lawrence “Larry” Roger Beck, now 65, is accused of murdering 20-month-old Michael Ray Sanders in an apartment in December nearly 43 years ago. A newspaper obituary listed the child as 18 months old at the time of his death.

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Officials said Beck was dating the child’s mother at the time of the murder.

CBS2’s Juan Fernandez said a cold-case unit in Louisville had been working on the crime for several months.

Beck reportedly had taken the child to the doctor’s office and said the boy had just become unconscious. The toddler died a short time later.

Fernandez said the autopsy at the time of the death revealed the boy had suffered traumatic injury.

Beck was arrested Tuesday by detectives with the Los Angeles Sheriff’s Department Major Crimes Bureau.

“Thankfully, the evidence from 1971 has stayed intact so we were able to go back and look at the evidence and determine what happened to the child,” Jefferson County Assistant Commonwealth attorney Diane Arnold told WLKY in Louisville.

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She added, “We have enough evidence to go forward.”

Prosecutors believe Sanders was shaken to death. Advancements in the study of shaken-baby syndrome and pediatric forensic medicine over the past four decades is credited with cracking the case.

Fernandez went to two addresses listed for Beck in Arcadia and spoke to neighbors who knew him.

On Oakwood Drive, residents told Fernandez they remember Beck as part of a quiet couple.

“Big surprise,” said neighbor Kermit Francis about the arrest, “I’d seldom see the man coming out of the house. I’d see the woman several times.”

The current resident on Longanrita Avenue said he’s still getting mail for Beck’s wife.

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Beck will be extradited back to Kentucky, officials said.