June 30, 2009 -- It was a route Lindsey Baum had taken many times -- a 10-minute walk down a densely populated suburban street between her house and a friend's.
But on Friday evening, the McCleary, Wash., 10-year-old left her friend's house for home and disappeared somewhere along the way.
"I think somebody took her," Melissa Baum said of her daughter, who was ready to enter the sixth grade in the fall. "I'm trying to constantly push away the bad thoughts."
Grays Harbor County Undersheriff Rick Scott said search efforts have escalated in the days since Lindsey's been missing from volunteers on foot -- still the most common tactic used -- to scent dogs, horses, all-terrain vehicles and helicopters.
"We're not ready to give up hope," Scott told ABCNews.com.
Baum said she last saw her daughter when Lindsey, along with her 12-year-old brother, Josh, headed out to Lindsey's friend's house in hopes she could get permission to spend the night at the Baum's house.
Baum said her children began squabbling over the use of Josh's bike on the way there and were stopped by a family friend who sent Josh home to end the argument. Lindsey continued on to her friend's house. When Lindsey's friend found out she couldn't stay the night, Lindsey headed for home around 9:30 p.m.
"When she wasn't home by 10, I started to get nervous," Baum said, adding that 10 p.m. is the curfew for her children.
She began calling Lindsey's cell phone, only to find that her daughter had left it plugged into the charger. Initially thinking that her daughter must have met up with friends in the neighborhood, Baum set out on foot to find her daughter.
But there was no sign of her. Eventually, her friend's parents joined the search by car. Baum even let her daughter's beloved German shepherd Kadence off its leash in hopes the dog would help find her. Finally, around 10:45 p.m., Baum said she called the police.
Baum described her daughter as outgoing, talkative and mature for her age. She loved to read and write and had big plans for her future.
"She insisted when she grows up she's going to be an author and an illustrator and a veterinarian," Baum said.
Police Search With Dogs, Check In on Sex Offenders
Scott said that, with the FBI's expertise with missing children, authorities have been conducting a simultaneous search and rescue operation with a criminal investigation, hoping to find leads.
The latter includes checking up on the resident and transient sex offenders in McCleary and neighboring communities and reviewing surveillance videos.
Scott said that there were a few businesses located just off the street Lindsey would have used to get home, and while the little girl did not appear in any of the videos, police have received clues about who was in the area at the time she disappeared.
Scott said witnesses were able to put Lindsey within a couple of blocks of her house just after 9:30 p.m. The last person reported to have seen her, he said, was a neighbor on her way to work.
Authorities had initially looked at the case as a possible runaway, but Scott admitted that "it's becoming less likely as time goes on."
Baum said she knows her daughter would not run away. She had been upset about her parents finalizing their divorce in April but had known it was coming, Baum said.
And Lindsey knew better than to go somewhere with a stranger -- it was something they had talked about previously.
"I think she's alive," Baum said. "I really and truly feel that strongly in my heart."
Baum said she and Lindsey had been planning to celebrate her upcoming July 7 birthday by going with a group of friends to the new "Harry Potter" movie a few days later.
Instead, the family is now trying to cope with a life without Lindsey. Baum said her son Josh is racked by guilt that he argued with his sister and left her to visit her friend on her own. And even Kadence is suffering the loss of a friend, refusing to eat normally.
Baum said she's baffled that no one has reported seeing anything the night Lindsey disappeared, unusual, since the street she took home is lined with homes.
"If we can find her," she said, "we can work through anything."