Whether he had anything to do with the murder or not, God knows! but he found out--nay, Middleton openly told him--that he had jewels of great value in his possession, which he carried about on his person.

Carr was the only person aware of that fact.

What follows?
Carr has Middleton's address in London.

Middleton goes to the house, and finds that his sister has moved to the next street.

That house to which he first went is broken into, and the poor woman in it is murdered, or dies of fright that same night.