It happened around 5:30 a.m. Wednesday at Broad and Spring Garden streets in Center City.
"Deranged, I mean, who would do something like that. We have maintenance people in here that early in the morning, they could have killed one of them," FOP spokesman Michael Trask said.
Trask says a witness watched a man throw the brick, set something on fire, and throw that in, too.
"I was expecting the worst. I was expecting the front of the building to be on fire and the fire trucks out here," Trask said.
However, there was no fire because the the Molotov cocktail didn't explode.
In the 40 years the FOP has been in this location, it has never had an incident like this one.
The union represents Philadelphia police officers and law enforcement retirees who are in and out of the buliding throughout the day.
Residents who walk past this building daily were disgusted to hear about the crime.
"The police is out there helping all of us. It doesn't matter what color you are, whatever. They're out there to help anybody and to go and do something like this is very disrespectful," Spring Garden resident Norman Jackson said.
It might sound ironic, but now this organization is asking the Philadelphia Police Department to step up police patrols in the area.
There is no surveillance video of the vandal who was wearing a dark hoodie and ran off through a nearby parking lot.
FOP says the crime only cost about $300 in damages.
The door was replaced with a tougher glass; it's now shatterproof.