District Attorney Kevin Steele joined Upper Dublin Township's police chief in Norristown to announce what they described as a significant arrest.
Tamir Hartsock, 23, of Glenside, is now in police custody, authorities said.
Steele said the suspect bought and sold 15 guns to who he described as "violent and dangerous people" over a two-year span.
Investigators said one of the guns sold was used in a quadruple shooting in Philadelphia's Lawncrest section on April 28.
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Police said three people were killed in that shooting, including two students.
A 15- and 16-year-old were both arrested that same day.
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Of the 15 firearms Hartsock purchased, authorities said only four have been recovered, meaning 11 are still unaccounted for.
Hartsock was arrested on charges of Unlawful Sale/Transfer of a Firearm, Dealing in the Proceeds of Unlawful Activities, Corrupt Organizations, Conspiracy, and other related crimes.