Police responded to the 400 block of Linden Street at about 5:20 p.m. when a woman called and said she found an explosive in packaging that should have had a cooking mold she ordered from eBay, police Sgt. Mike Andraychak said.
Arriving officers found a vintage 40-millimeter antiaircraft mortar projectile and evacuated the woman's building.
The two buildings on either side of the woman's were also evacuated, and an ambulance was parked down the street as a precaution, Andraychak said.
Investigators determined that the device was essentially harmless.
"The projectile didn't have insides that would cause it to explode," Andraychak said.
The U.S. Postal Service is conducting its own investigation into the issue. No arrests have been made.
No one was injured in the incident.