Dawn Nguyen, 24, is believed to have helped 62-year-old William Spengler buy an AR-15 rifle and a shotgun. It is not yet known exactly how Nguyen knew the shooter.
Spengler, who took his own life after the shootings, could not buy the guns himself, as he was a felon convicted of killing his grandmother decades ago.
The firefighters were responding to a fire that began in Spengler's home in Webster, N.Y., and ultimately burned down seven houses.
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