And to study them she must question till she found motive.
Why had Harold sent her the letter?
His own words said that it should be in her hands.

Then, again, he said it might comfort her to know the letter was safe.

How could it comfort her?
How did he get possession of the letter?
There she began to understand; her quick intuition and her old knowledge of Harold's character and her new knowledge of Leonard's, helped her to reconstruct causes.

In his interview with her he had admitted that Leonard had told him much, all.

He would no doubt have refused to believe him, and Leonard would have shown him, as proof, her letter asking him to meet her.