Difficult film search: 50s/60s old crime/thriller (with quote) wanted

Hello you all,

I'm probably looking an us production, for discerning crime film lovers, sought-after film is under the motto: "Two lies that result in one truth"
The suicide note from a long-dead woman reveals the following: her daughter had been murdered by her husband (i.e., the writer's son-in-law); but he was acquitted. When the mother fell ill (and probably saw her end near), she poured poison into a water bottle, wiped her fingerprints off and asked her son-in-law to serve her a glass of water. He was then sentenced to death, the discovery of the letter came too late for him.
Closing words (voice of the older woman): "Two lies that make up one truth."

I am grateful for reading any kind of help.

greeting

Maximilian