What did the police show mr. utterson that convinced him that the murderer was mr. hyde and connected to dr. jekyll?

The night of the murder of Sir Danvers Crew, which was a member of parliament, the police decide to contact Mr. Utterson to oversee the case. Utterson, suspecting of Mr. Hyde goes to his apartment but is unable to find him nor any signs of where he could have gone. He then decides to visit Dr. Jekyll. Jekyll tells Utterson that he has no longer any relationship with Mr. Hyde and shows him a letter that he sent him in which he apologizes for the troubles he could have caused.

That night, Utterson's clerk, who also saw the letter, states that the handwriting in the letter is very similar to that of Dr. Jekyll.