A California man has been arrested for allegedly beating to death his girlfriend's four-year-old daughter, who had Down Syndrome.

According to a press release from the San Bernardino Sheriff’s Department, on Tuesday night 24-year-old Adam Hughes was babysitting his girlfriend’s children at their home in the 10000 block of Kernwood Court in Rancho Cucamonga when four-year-old Kylee Willis 'angered' the man by interrupting him.

The sheriff’s department wrote that the 5-foot-11, 220-pound Hughes assaulted the special needs toddler and then put her to bed.

Adam Hughes, 24 (left), has been charged with murder and willful cruelty to a child in the beating death of four-year-old Kylee Willis (right), who had Down Syndrome

Hughes was babysitting the children of his girlfriend, Corinne Blake (pictured together), whe nKylee allegedly angered him by interrupting him

After allegedly assaulting Kylee (pictured in his arms), Hughes put her to bed

When he went to check on Kylee several hours later, he found her unresponsive. By that time, the toddler's fingers and lips had turned blue.

Hughes then called his girlfriend and the girl’s mother, Corinne Blake, who dialed 911 just before 8pm.

Paramedics transported the four-year-old victim to San Antonio Medical Center, and from there she was airlifted to the Loma Linda University Medical Center to be treated for severe head trauma.

Kylee spent several days on life support and succumbed to her injuries on Friday 'as a result of the injuries caused by Hughes,' according to the press release.

Hughes and Blake have been together for two years and got engaged to be married last May

Both Hughes and Blake were initially arrested on a charge of willful cruelty to a child, but after Kylee's death, the boyfriend was slapped with an additional charge of murder.

He is currently being held without bond pending his next court appearance scheduled for Tuesday.

Blake, meanwhile, was released on Thursday after posting bail.

According to her Facebook page, Blake works as a cashier and has four children: two boys and two girls. She and Hughes have been dating for two years and got engaged in May of last year.