Read the excerpt from The Strange Case of Dr. Jekyll and Mr. Hyde.

. . . and the next moment, I had sprung to my feet and leaped back against the wall, my arms raised to shield me from that prodigy, my mind submerged in terror.

"O God!" I screamed, and "O God!" again and again; for there before my eyes—pale and shaken, and half fainting, and groping before him with his hands, like a man restored from death—there stood Henry Jekyll!

This part of Lanyon’s letter helps the reader understand that

Lanyon knew that Hyde killed Jekyll.
Lanyon is now Hyde’s prisoner.
Jekyll is trying to hurt Lanyon.
Jekyll and Hyde are the same person.