The girl remains hospitalized in serious but stable condition. She has spoken to detectives on two occasions, but a suspect description is not yet available.
Authorities have narrowed down the time that they believe the incident occurred - sometime between 11 p.m. Saturday and 1 a.m. Sunday.
Police say the mother was at work while her three kids - ages 12, 9 and 3 - were at home. The 9-year-old was asleep when an unknown man entered through an unsecured window and kidnapped her from their apartment on Herman Drive and Pike Street.
Not long after the kidnapping, the girl was found wandering in the streets about three miles away, knocking on doors and asking for help. She showed signs that she had been assaulted, but police are not saying how.
Police believe abduction was an isolated incident, but it's done little to calm the nerve of residents in the area.
"I've been a parent on high alert since Sunday," said Steve Mundy of Riverside.
During a news conference, police said there's no reason for residents to be more concerned than any other time.
"I would tell people the same thing I tell them every day. They need to keep their doors and their windows locked. They need to be vigilant about what's going on in their neighborhood," said Det. Roberta Hopewell of the Riverside Police Department.
Authorities said they are looking at surveillance video of a dark-colored truck that was seen in the area, but they are unsure if the truck in involved in the abduction.
The two siblings of the victim are staying with a relative at an undisclosed location. The mother of the victim is being cooperative with the investigation, but it is unknown if she will face any charges.
Anyone with information on the abduction is urged to contact Riverside police at (951) 353-7124.