Federal authorities say a woman has been charged with illegally buying guns used in the killings of three Minnesota first responders in a standoff at a home in the Minneapolis suburb of Burnsville. Officers Paul Elmstrand and Matthew Ruge, and firefighter-paramedic Adam Finseth were slain during the Feb. 18 standoff.Their memorial service two weeks ago drew thousands of law enforcement officers, firefighters and paramedics.Investigators say a man opened fire without warning after lengthy negotiations, then later killed himself. There were seven children in the home at the time.