Law enforcement in the town of Valley Springs are warning residents that the man they're searching for is considered armed and dangerous. They encouraged residents to keep their doors locked.
Local TV station KCRA says the victim's 12-year-old brother encountered the intruder in his home at around noon Saturday and saw the man run away.
The boy went to check on his sister, Leila Fowler, and found she had been stabbed. The girl was later pronounced dead at a hospital.
Initially Leila was reported as being 9-years-old, but Coroner Kevin Raggio said Sunday that she would have turned 9 in June.
The Associated Press contributed to this report.