The incident happened Friday around 7:30 p.m. on the 300 block of W. Warren Street.
Officials say once police arrived, two teens were located suffering from gunshot wounds.
They were pronounced dead at the scene.
Authorities say an investigation determined the two teens attempted to rob the shooter as he walked to his car from an area residence.
They say a struggle ensued, and that's when investigators say the man pulled out his own, legally registered gun and shot his attackers.
The man then called 911 and rendered aid until police arrived.
Officials say a ghost gun with an extended 30-round magazine was recovered from the scene.
That man was taken into custody Friday night, but he was later released after questioning and has not been charged, according to Montgomery County District Attorney Kevin Steele.
An autopsy was performed Saturday by the Montgomery County Coroner's Office.
Officials determined that one of the teens was shot five times-- and the other teen was shot once in the torso and once in the finger.
"We have a lot of work to do on this investigation," Steele said in a statement.
Anyone with information is asked to contact the Norristown Police Department at 610-270-0977 or call the Montgomery County Detectives' Tip Line at 610-278-DOIT (3638).