15 buses belonging to the Philadelphia School District were damaged among the 180 on the lot. Three buses were destroyed and 12 sustained either major or minor damage.
Windows on the buses were smashed out at the depot on the 3800 block of Woodhaven Road.
Officials found the additional damage while investigating the weekend fire. In that case, police believe two buses were set on fire, while another three were also damage by the flames.
Firefighters found five separate fires when they reached the lot.
Investigators suspect the fire was the work of an arsonist.
Ralph Brooks lives across the street from the lot where police spent most of Monday morning investigating.
"My kids are all grown so I'm not worried about the transportation part, but it's still taxpayer money," Brooks said.
The district says there are no surveillance cameras watching the buses, only roving school police.
More than a year ago, a similar act of vandalism happened at the same lot.
Neighbors say down the street there is a gate that remains locked, but a closer one is always wide open.
District spokeswoman Jamilah Fraser, however, says there is a lot of security at the lot and it will continue.
At this point, investigators are not sure who's behind the vandalism.
"Our number one goal right now is to find out who did this and make sure something like this doesn't happen again," Fraser said.