The mother of a 13-year-old girl missing since Thursday says she regrets making the emotionally troubled teenager walk home alone to cool down, only to never see the daughter make it back to the residence.

"It was a bad choice on my part," Shannon Tanner told the St. Louis Post-Dispatch on Monday from her home in this eastern Missouri community, four days after Bianca Nicole Piper (search) disappeared.

The search continued Tuesday for the teenager, who vanished after Tanner drove her about a mile from home and dropped her off, hoping the walk would give the girl an opportunity to calm down.

Tanner said mental-health counselors had recommended the tactic to her in raising the daughter, who has been in counseling since she was 4. The girl suffers from attention deficit and bipolar disorders (search), and takes several medications to help control her mood swings and aggressive tendencies, the mother said.

Tanner said she first tried letting Bianca walk off her anger Wednesday night after the teenager became upset about doing homework. Bianca walked straight home, commenting that the walk went quickly, the mother said.

So on Thursday, when Bianca said she didn't want to do the dishes, Tanner dropped the girl off down the gravel road — a little past where she let her out the previous night. Given that it was after 6 p.m. and close to nightfall, Tanner gave the girl a flashlight.

Tanner said the girl said nothing harsh, and that "she never said she was going to run away."

"She was more calm than she had been the night before," Tanner said.

Tanner pulled into a driveway, then watched Bianca to make sure she was heading home. After a green sport utility vehicle passed, Tanner drove back to Bianca and told her not to take a ride from anyone. The mother then went home.

"I wish now that I had told her that I love her," Tanner said.

When Bianca didn't return by 6:40 p.m., Tanner by car retraced her route, failed to find the girl and searched door to door with her boyfriend. About 7:30 p.m., they notified authorities.

"I was worried before then, but when it started getting dark and snowing, I got really scared," Tanner said. "I thought, 'My God, it's freezing out there, and all she's got on is a hoodie."'

Tanner said relatives believe Bianca may have left the area, given that there's been no trace of her in the extensive search. They have begun circulating fliers about Bianca in Canada and several states.

Searches in recent days had included hundreds of people on foot, four-wheelers and helicopters.

Tanner said Bianca is naive, reads at a second-grade level and in many ways is a typical seventh-grader, a fan of pop artists Hilary Duff and Britney Spears. The teenager likes to draw pictures and loves animals, her favorite being horses.

The 5-foot-6, 185-pound girl, who has brown eyes and curly brown hair, was last seen wearing blue jeans, white tennis shoes with purple stripes, and a gray Adidas hooded sweatshirt.