YUMA, Ariz. (AP) -- A man already serving a life prison sentence for a 2007 murder has been accused of killing six people in Yuma two years earlier. Preston Alton Strong has been indicted by a Yuma County grand jury on six counts of first-degree murder and one count each of robbery and burglary. Police say Adrienne Heredia, 29, and her four children ranging in age from 6 to 13 were found dead inside a Yuma house on June 24, 2005. Luis Rios, 35, also was found with multiple gunshot wounds in the home's backyard and later died at a Yuma hospital. Police say witnesses reported seeing a man walking back into the house after the shooting. Strong is serving a life prison term with no chance for release for killing a Yuma physician.