It happened at approximately 6 p.m. on Thursday.
Surveillance video shows the suspects letting themselves inside the building and digging through several boxes in the mailroom.
According to the property manager, the suspects used a USPS mail key.
"It's not really just a concern for us, it's a concern for every building in the area that one key gives access to. I don't know how many units could potentially be hit," said Property Manager Jorden Leibowitz.
Authorities say the postal inspector is aware and is investigating this incident alongside the Philadelphia Police Department.
Anyone with any information about this incident or who may recognize the suspects is asked to contact the police.