Terrance Matthews remains in jail, accused of killing his former girlfriend's 9-year-old brother, Ishon Mathlin Jr. and their grandmother, Jennifer Vassell, 68, on June 20.
Authorities say both victims were stabbed multiple times and Ishon had been drowned in a bathtub.
Matthews led police on a chase and live-streamed it before being arrested.
According to the Burlington County Prosecutor's Office, the motive for the killings stem from an argument Matthews and his ex-girlfriend had during a telephone call earlier in the day.
Matthews was charged with two counts of murder, kidnapping and other related offenses.
After that arrest, police said he was connected to and charged him with the February slaying of a man who was sitting in a parked car in Willingboro.