One victim had gone to a gas station to buy a soda. Another was sitting in a car with a friend. A third was taking his three children to buy a puppy.

Prosecutors said this week that a Chicago man, Antonio Reyes, fatally shot six people in the city over a nine-month period in 2020, beginning when he was 16 years old.

In all but one of the cases, Mr. Reyes seemed to have no connection to his victims, and was targeting “complete strangers,” the Cook County state’s attorney, Eileen O’Neill Burke, said in an interview on Thursday.

“It appears he was just doing it for the thrill of it — to murder people, repeatedly,” she said.

Mr. Reyes, now 21, has been in a Cook County jail since he was arrested in December 2020 and charged with one of the murders. Prosecutors said on Wednesday that he had been charged with five additional murders and four attempted murders, all occurring in 2020.

The police said Mr. Reyes had been linked to the killings through shell casings from a .40-caliber handgun, social media posts and other evidence. Mr. Reyes has also been charged with using a shank to try to kill his cellmate in jail, Ms. Burke said.

Mr. Reyes’s lawyer did not immediately respond to a request for comment on Thursday. Mr. Reyes was arraigned on five charges of murder and four charges of attempted murder on Tuesday and pleaded not guilty, the state’s attorney’s office said.