Sheriff's deputies and paramedics were called to a home in the 4100 block of Lemonberry Place in Thousand Oaks on the morning of Nov. 29.
They found a 4-year-old girl that they say was suffering from "severe injuries caused by physical child abuse."
Firefighters performed CPR and then rushed the girl to a local hospital, where she was listed in critical condition.
Investigators spoke to the girl's father, 25-year-old Tekquan Alexander, and then arrested him for mayhem and felony child abuse.
They also executed a search warrant at the home. They say they found an unsecured assault rifle and a handgun in Alexander's bedroom.
Alexander is prohibited from possessing firearms, according to the Ventura County Sheriff's Office, so he was also booked for weapons violations.
Alexander was booked and is being held on $500,000 bail.