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For more than 20 years, the murder of Howard Pilmar, a high-end coffee shop and office supply entrepreneur who was stabbed dozens of times and left in a hallway outside his Manhattan office in 1996, went unsolved.

From the get-go, the police investigating Mr. Pilmar’s grisly death were convinced that the culprit was someone who knew him. As they investigated, they discovered that Mr. Pilmar’s wife, Roslyn, and her brother, Evan Wald, had both visited Mr. Pilmar’s office the night before he was found dead.

The police learned that Mrs. Pilmar stood to receive more than a million dollars in life insurance benefits after her husband’s death. When they questioned Mr. Wald, they noticed a cut on his hand.

But as the years passed, detectives never found enough evidence to charge or arrest Mrs. Pilmar or Mr. Wald. Yet they could not find any other suspects in Mr. Pilmar’s murder either.