A Philadelphia high school was put on lockdown following an armed robbery Friday.
Police say Jules E. Mastbaum Area Vocational Technical High School at 3116 Frankford Ave. in the Kensington section of the city was put on lockdown after a student was robbed at gunpoint.
Shortly after the school's 600 students were dismissed for the day, two students and a masked gunman approached the 17-year-old victim at his third floor locker where they assaulted and robbed him, according to police.
The district knows who the two student attackers are, but are unsure who was wearing the mask.
Investigators are still searching for the suspect who pulled the gun.
"The third individual...was armed with some type of handgun, whether it was a real handgun, a simulated weapon, a pellet gun," said Philadelphia Police Chief Inspector Carl Holmes, "We do not know at this point."
Mastbaum has metal detectors at the building's front doors - the only location where students are permitted to enter, according to school officials.
But officials say students exit through all doors during dismissal, which may have provided the gunman with an opportunity to pass into the building undetected.
The suspects stole a cell phone, portable video game system and other electronics from the victim, according to police.
Authorities are checking school surveillance cameras that may have caught the incident.
Students could be seen leaving the building as the lockdown was lifted around 4:30 p.m.