– In the grainy video, Zhang Xiuhong can see her daughter ride her bike down a country road on her way to school one spring afternoon six years ago.

In the next shot, Yao Li rides down a driveway a few moments after her classmates walk by. Then, the pictures stop: The 15-year-old disappeared just minutes after that surveillance footage was taken, leaving only a shoe as a clue in a nearby ditch.

Zhang and her husband have since searched all over China for Yao Li, hoping to rescue her from a child trafficking industry that swallows up thousands of boys and girls every year. Along the way, the couple have also been harassed, arrested and jailed repeatedly by police who accuse them of stirring up trouble by joining with other parents and taking their search to the streets.

"We go out and search, and then all these police surround us," Zhang said in the dingy room she and her husband share near where her daughter was last seen. "Nobody's watching for my daughter. Nobody's doing anything. How can we have any more hope?"

In a tightly monitored society where authorities detain even relatives of air crash victims demanding government action, Zhang and other parents of missing children have learned that they must fight on two fronts.

First, they're up against a sprawling, opaque network of abductors and illegal buyers and sellers of children. And since police efforts to find children often leave parents unsatisfied, they must negotiate with authorities to hunt for the kids themselves.

As many as 70,000 children are estimated to be kidnapped every year in China for illegal adoption, forced labor or sex trafficking, making it one of the world's biggest markets for abducted children, according to the state-run newspaper China Daily. By comparison, in the U.S., about 100 children are abducted annually by people who are strangers to them, said the Polly Klaas Foundation, a nonprofit dedicated to preventing crimes against children and assisting in the recovery of missing ones.

The U.S. State Department said in its annual trafficking report this year that China "does not fully comply with the minimum standards for the elimination of trafficking."

Chinese authorities have tried to show they're tackling the problem, including launching a special anti-kidnapping task force in 2009, which government media say has busted 11,000 trafficking gangs and rescued more than 54,000 children across the country.

In October, the issue was highlighted in the Chinese-produced movie "Dearest," which told the true story of a couple who found their abducted son after searching for three years.

Still, many parents say they toil largely on their own, with the police at best leaving them alone.

Xiao Chaohua, whose son was 5 when he disappeared outside his shop in 2007, said appeals to government-run TV to broadcast pictures and names of individual children are largely rejected, as are suggestions to develop a Chinese version of U.S. Amber Alert warning systems to spread information about missing children through roadway signs or other means.

"They won't broadcast it because if they do, it'll expose one of China's problems — the fact that children go missing here," Xiao said.

The Public Security Ministry, which runs the anti-kidnapping task force, did not respond to several phone calls and a fax seeking comment.

According to Pia Macrae, China director for the international nonprofit group Save the Children, Chinese police are often more willing to help families with greater means and even then frequently don't tell parents what they're doing.

About 1,000 families have formed a Beijing-based support group that shares leads about missing children and negotiates with police to allow parents to search for their children. They often go to cities where child and sex trafficking rings are reported to be operating and try to track down suspected traffickers.

Over the past six years, the group has found two children, both of them abducted from small cities and sold to adoptive families, Xiao said. The group found one boy in an orphanage in central Henan province, rejected by his purchasers because of a heart condition and just days from being sent overseas for adoption.

After China toughened its anti-trafficking laws in 2009, prices for abducted children shot up as much as tenfold to $32,000 for boys and nearly $10,000 for girls, he said. Children considered particularly attractive fetch even higher prices.