Sarah Ridgeway said the discovery of the backpack and water bottle gave her "a sliver of hope" because she thought that if something "really bad" had happened to her daughter the bag would not be sitting in plain sight.
The Colorado Crime Lab is processing the backpack for possible DNA evidence.
The search for Ridgeway expanded Monday with more than 100 people combing roadsides and rocky terrain. Meanwhile, police are checking reports that three weeks ago a suspicious man was seen approaching children.
"It's a piece, but at this time we have nothing to say that is related to the disappearance of Jessica," Westminster Police Department spokesman Trevor Materasso said.
Police and FBI agents are still urgently going door-to-door in Ridgeway's neighborhood asking people to search their vehicles, yards and homes. Police asked a neighbor named William Kesel to let them search a locked room in his home to make sure the missing girl wasn't in there.
"They had to rule us out, and they got to rule that room out," Kesel said.
ABC News obtained exclusive video this morning that Ridgeway filmed herself last May playing with a video camera and the family dog.
"That's where the camera is. See?" she can be heard saying. "Right there, look. That's the camera."
The parents both said that they had no idea who could have wanted to harm their little girl and thanked authorities and volunteers for all of their help.
The family said they are holidng on to hope and have prepared Jessica's room for her return, complete with the "nest" she likes to build on the top bunk of her bunkbed -- a cozy structure made of a sleeping bag and stuffed animals.
When asked what message she would like to send her daughter, Sarah Ridgeway said, "You know that your family is strong. We'll be there forever and we will do anything to find you. You are strong. We know you're strong and you can fight this and we are fighting for you, too. And we love her and we'll never stop loving her no matter what happens. We'll lover her forever and ever. We just want her home."