Whoops.
I'm thinking of:
Leopold and Loeb
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Murder of Bobby Franks
Leopold (then 19 years old) and Loeb (18) settled on the kidnapping and murder of a young boy as their perfect crime.
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Police found a pair of eyeglasses near the body. Though common in prescription and frame, they were equipped with an unusual hinge mechanism purchased by only three customers in Chicago; one was Nathan Leopold. When questioned, Leopold offered the possibility that his glasses (now owned by the Chicago History Museum) might have dropped out of his pocket during a bird-watching trip. The destroyed typewriter was discovered soon thereafter.
The two men were summoned for formal questioning on May 29. They asserted that on the night of the murder, they had picked up two women, Edna and May, in Chicago, using Leopold's car, then dropped them off sometime later near a golf course without learning their last names. Their alibi was exposed as a fabrication when Leopold's chauffeur told police that he was repairing Leopold's car that night, while the men claimed to be using it. The chauffeur's wife later confirmed that the car was parked in the Leopold garage on the night of the murder.