They say the pictures on his mobile phone connected him to the weapon.
With a large mural of Dr. Martin Luther King, Jr. as a backdrop, Chester community leaders stood together just hours after Avery was charged.
"You've destroyed two families with an 8-ounce bullet when it should have never even happened," said Chester Mayor Wendell Butler, referring to Avery.
There were nine victims at the birthday celebration which had more than 200 people thanks to a mass invitation that went out on Facebook.
18-year-old Robel Laboy and 19-year-old David Johnson died from their injuries. In addition to Avery, two other people have been charged in connection with the party and shooting.
Delaware County District Attorney Mike Green says officers found Avery holding the gun used in the shooting. Investigators say when they searched the teen's phone later they found pictures of the same weapon.
"That firearm we believe was in his possession for a number of weeks prior to the incident last Friday night," said Green.
The mass shooting sparked outrage in Chester, which has suffered an increasing amount of youth violence this past year.
The judge set bail at a staggering $2.5 million for Avery who was sitting in jail on lesser charges of aggravated assault. Police say getting information from the public was tough at first. But this week more than 100 tips rolled in.
Kenny Covert attended Johnson's funeral today. Covert leads this group of men called Brothers of Concern and says it is time for more people in this community to step up.
"We've got to come together," he told Action News. "Until we all take ownership of this problem, it's still going to be a problem."
The working theory is that the shooting was sparked by ongoing arguments between two groups, but the District Attorney couldn't say if the incident was gang-related.