RANCHO CUCAMONGA, Calif. (KABC) -- A Rancho Cucamonga man has been arrested for allegedly causing the death of his girlfriend's daughter, who had Down syndrome.Authorities say Adam Hughes, 24, of Rancho Cucamonga, was watching his girlfriend's children Tuesday night when one of them interrupted him, making him angry.He allegedly assaulted the girl - 4-year-old Kylee Willis - and then put her to bed.Hours later he noticed Kylee was unresponsive, with blue fingers and lips. He called his girlfriend, Corrine Blake, and she then called 911.Paramedics brought Kylee to a local hospital. She remained on life support for several days and then died on March 29.After the initial injury, police arrested both Hughes and the girl's mother, Blake, for willful cruelty to a child.After Kylee died, Hughes was also charged with murder. He is being held without bail and has a court date set for Tuesday.