Patrolman John O’Brien heard the shots and hurried to the scene. Mahnken had gone upstairs to his apartment above the store and O’Brien followed, confiscated Mahnken’s pistol, and took him to the station. Mahnken admitted that he shot Steffens and said he had done it because Steffens had broken up his family and caused trouble between himself and his wife. He felt he had to do something to protect his children.
Though Mahnken and Steffens had been friends, it was well known in the community that Mahnken had, for years, accused Steffens of paying too much attention to his wife. It was just as well known that Mahnken’s accusations were completely unfounded. A niece of Mrs. Mahnken, Annie Doscher, was living with them and she had been the object of Steffens attentions. The two were in love and at the time of the murder were planning their wedding. Diedrich Mahnken was convinced that this was a ruse and that Steffens was actually coming to see his wife. Though Mrs. Mahnken did everything she could to prove her husband’s suspicions were wrong, he would not let them go. She finally convinced Annie to move out. Steffens no longer stopped by the Mahnken’s home and for a time there was peace.
While seemingly rational in his everyday affairs, the common belief was that Mahnken had become insane on the subject of his wife’s alleged intimate relations with Steffens. This sentiment was echoed by the physicians who attended him in jail. Incarceration seemed to aggravate Mahnken’s condition; he began laboring under the delusion that his friends were trying to poison him and he was very careful about what he would eat.
In June 1883, Deitrich Mahnken was declared insane by Kings County Judge Moore. He was sent to the State Insane Asylum in Utica, New York.
"A German's Fatal Jealousy." New York Herald, 18 Apr 1883.
"A Husband's Mad Deed." The National Police Gazette, 5 May 1883.
"Is He Insane?." New York Herald, 25 Apr 1883.
"Jealousy And Murder." Truth, 18 Apr 1883.
"Suburban Notes." New York Herald, 30 Jun 1883.