PHOENIX - An Arizona woman found dead inside her Phoenix-area home with the bodies of her 17-year-old son and 12-year-old daughter is believed to have shot her two children to death before turning the gun on herself, authorities said on Tuesday.The scene of the apparent murder-suicide was discovered after a family friend called authorities to report suspicious activity at the house, spotting a parked car with a hose running from the tailpipe into a window of the home, Maricopa County Sheriff Joe Arpaio said."We're pretty sure that the mother killed her kids and then herself, at this point," Arpaio said in a brief telephone interview. "This is so sad."Arpaio said sheriff's deputies donned protective clothes before entering the property and quickly located the three bodies, all of whom had suffered gunshots, Arpaio said.A handgun was found near the body of the mother, identified as Lisa Gerhart, 45, he said. The names of her slain son and daughter were not made public because they were minors.Investigators believe that Gerhart and her husband were having marital problems and may have been living apart at the time, Arpaio said. The husband was not home at the time.A final determination on the cause of death will be made by the county medical examiner.