It is unknown when he began to murder women, but during World War I, Grossmann sold meat on the black market and later owned a hot dog stand at a train station near his home. It was strongly believed he used the flesh from his victims as meat substitute which he sold to the unsuspecting public and threw the bones and other inedible parts into the river. He would often invite prostitutes to his apartment for sex, whilst homeless women were offered food for the same arrangement, as well as advertising for single women to work for him who were all then butchered, before he disposed of the remains. This led to a spate of missing persons cases beginning in May 1918 when several bodies began showing up in the Luisenstadt Canal and Engelbecken Reservoir in various stages of decomposition.
In October 1920, 33-year-old Freida Schubert went missing. She had travelled to Berlin from Dresden, and on the day of her disappearance she had been seen propositioning many men, when one eventually accepted her services. Between October 7/9, 1920, the remains of a young women were found in the Luisenstadt Canal, which were later identified as those of Freida Schubert. On 16 October the Berliner Morgenpost reported that the killer had sawn through her bones with such brutality that he arm had been pulled from the shoulder and her heart had been pulled from her ribcage.
The police believed it was the work of a sadist and began by questioning any potential witnesses, one of whom said they saw the young woman in the company of Carl Grossmann. Police then made a search of his apartment on October 21, and found the missing woman’s handbag, however Grossmann was able to offer an innocent and plausible explanation and the matter was pursued no further. In December 1920, a young woman known as Melanie Sommer vanished, and the rash of unexplained disappearances and bodies discovered by police continued throughout 1921 right up to early August, when Elisabeth Barthel disappeared which saw the number of cases of missing women reported to the Berlin Police Department rise up to twenty three.