Who is Lucius and Marcus Isaacson from The Alienist and what is their importance?

Lucius and Marcus Isaacson are detectives with the New York City Police Department. If not for Roosevelt, these two might not have been hired because they are Jewish and Jewish cops were not often hired under the previous administration. The Isaacsons also have a strong background in forensic medicine, including knowledge of fingerprints, handwriting analysis, and medical investigation. If it were not for the Isaacsons, the investigators might not have learned that the killer left fingerprints on his first victims and at several of the latter crime scenes. Also, the Isaacsons are able to look at a letter the killer sent to Mrs. Santorelli and learn that the killer is only in his thirties based on the style of penmanship he learned in childhood. The Isaacsons are the true detectives of this novel, providing clues that Kreizler is able to translate into a psychological profile.