Plymouth District Attorney Timothy Cruz said Tuesday autopsies found the manner of their deaths was homicide.
The 25-year-old woman's body was found stuffed in a duffel bag in the trash bin Sunday night after police received an anonymous phone call. Her toddler son's body was discovered near hers.
The woman was an immigrant from Ecuador. She last was seen alive Thursday at her home in Brockton, a city of about 100,000 residents just south of Boston.
Investigators don't know when the bodies were dumped in the trash.
No one has been arrested. Prosecutors say these weren't random killings.