The knife was found around 5:00 p.m. Sunday in the yard of a home on 12th Street in Hammonton.
The murder happened at 32 S. Madison Avenue, which intersects with 12th Street.
Investigators say that knife has not been confirmed to be the weapon used in the stabbing deaths of 64-year-old Diana Patterson and her 29-year-old son, Ryan.
Ryan Patterson had more than 20 stab and slash wounds, authorities said, while Diana Patterson had more than 10 stab and slash wounds.
The bodies were found inside the home early Thursday morning.
Meanwhile, on Monday, several warrants were issued in Deptford Twp., Gloucester County and several items were seized. However, authorities wouldn't say what, if anything, was found.
There have been no arrests and the Atlantic County Prosecutor's Office had no comment on any possible suspects.
Anyone with information is asked to contact the Atlantic County Prosecutor's Office at 609-909-7800 or Crimestoppers at 609-652-1234 or 800-658-8477.