The mother of five remembers the moment when everything she thought she knew about raising her children exploded into shards of anger, despair and confusion.She was sitting at a conference table with federal computer forensics experts, a victim’s advocate, a police detective, and state and federal prosecutors. They showed her photos she couldn’t have imagined: her ex-husband, naked, posing next to their 7-year-old daughter, who was unconscious on his bed.

“I asked for a garbage can. I thought I was going to throw up,” she says. “Those photos are burned in my head.”

She learned that her ex-husband had drugged and raped their daughter and invited other men to choose among his three young daughters for sex. He took photos of himself and another man raping the youngest and posted them online for other pedophiles to enjoy.

If not for the agents who knocked on her door that Fourth of July weekend in 2010, the mother would have had no idea. “They swooped in and saved my kids,” she says. “They were my kids and I did not know they were in danger.”

The expansion of the “Dark Web,” where pedophiles hide using websites that encrypt their computers’ identifying information, has fueled an explosion of child pornography that has law enforcement in a race against time to find victims before they are abused again.

Investigators follow the trail of images around the world any way they can. They rely on traditional detective techniques, such as interviewing suspects, but they also use modern-day digital methods. They enhance blurry backgrounds for clues to a photo’s location. They monitor websites popular with pedophiles. They use social media to blast photos of suspected child pornographers in the hope that someone will recognize them.

“There’s no more vulnerable person than a child,” says Jim Cole, who heads the child victim identification program in the federal government’s Cyber Crimes Center at Homeland Security Investigations. “The first goal is to find the child and prevent further harm.”

Cole spends his days on the third floor of a nondescript building in a nondescript office park in suburban Washington, D.C., poring over thousands of sexually explicit images of children.

Finding the victims is urgent, he says, because 80% of child pornography victims are abused by someone they know, most often a parent, guardian, relative or family friend. That means the abuse is probably ongoing.

“The nature of child pornography is that this is not ‘stranger danger,'” says Lauren Schuette, who manages the Child Victim Identification Program at the National Center for Missing & Exploited Children. “We may not know who they are, but we know they are in a home and they are with the people they trust the most.”

“HOW COULD THIS HAVE HAPPENED?”

The center, a clearinghouse for information about child victims, works with police to identify children. It has helped identify some 5,500 children — out of tens of thousands.

Child pornographers have a compulsion to trade images and videos like baseball cards. The more graphic the image, the more they trade it. That leads investigators to a warren of cases where the arrest of one pornographer leads to another who leads to another.

The case of the girl and her father followed that pattern, according to court documents and the lead federal agent on the case. It came to light when a child pornographer arrested in Buffalo, N.Y., led to another in Milwaukee, who led to the father.

(USA TODAY does not name victims of sexual abuse. To protect the victims in this story, the names of the victims, their mother and the defendants, and the state where they lived, are being withheld.)

In the spring of 2010, Reynaldo Franklin had been working five years for Homeland Security Investigations, a division of Immigration and Customs Enforcement.

An agent in Milwaukee called him to say he’d arrested a man for child pornography who said another man in another state offered him the chance to have sex with one of the man’s daughters. The Wisconsin man told agents the father sent him more than 100 sexually explicit photos.

Unlike most child pornographers, the father shared his real name, the names of the daughters and the town where they lived.

“He took a huge risk,” Franklin says, and it got him nabbed.

Franklin contacted local police to see if the father had ever been arrested. He hadn’t. He checked the town’s property records and found that the father owned a home. He checked with the school district and found photos of the girls in their yearbooks, matching them to the sexually explicit images he received in a DVD from his Milwaukee counterpart.

The most graphic photos were of the youngest daughter, the one who was raped. The other girls were shown in bathing suits or at parties.

When federal agents arrested the 47-year-old father, they found a cache of child porn. The collection included photos of the father raping his daughter, and one more surprise: pictures of another man who had taken the dad up on his offer. They showed that man committing sex acts on the same unconscious little girl.

The father told investigators that he gave the daughter Ambien, a strong sedative, so she has no memory of the assaults.

Sitting in the conference room with the agents and prosecutors, the mother tried to grasp the details. She threw herself on the table and sobbed.

“I questioned myself,” she says. “How could this have happened? How did I not know? Why would someone do this to my child?”

The mother says she had no reason to suspect her ex-husband of abusing their children. He appeared to be a model dad. She says the girl showed no physical or emotional signs of abuse when she returned home from visits with her father.

She never suspected the twisted side he acknowledged to investigators when he told them he had always been attracted to little girls.

“This,” she says, “was a person I had complete trust in with the children.”

A SECOND CHANCE

For every child victim saved, thousands more are unidentified.

Sometimes investigators get a second chance.

That was the case with two young girls seen on a set of images found on the computer of a man arrested in Los Angeles in 2011. A year later, photos and videos showing two young girls being abused were confiscated from a Denver man’s computer. One video showed a man raping a girl who appears to be between 7 and 9. The girl is crying as a woman tells the girl, “It’s OK. It’s no big deal.”

Analysts at the National Center for Missing & Exploited Children recognized the girls from a series of images they already had in their database. They contacted Cole at Homeland Security Investigations.

He enlarged the photos searching for clues to the location or abusers. In the background of some photos, he found military items, including a canteen cup and a field locker. He turned to the Department of Defense, but the items were too commonplace. He found a hunting magazine that helped investigators narrow down when the photos were taken.

None of it got him closer to identifying the girls.

One of the photos, though, showed the face of the woman. Investigators released that photo nationwide.

A friend of Michelle Freeman saw it on a news story posted on Facebook, and she and other friends commented that they recognized her. One of those friends phoned Freeman, 40, and her husband, Michael, 39, of Salem, Ore.

Two weeks later, the couple turned themselves in to local police, who weren’t aware of the federal case.

“The local police call me,” Cole remembers, “and the first thing I ask is, ‘Where are the kids?’ and the officer says, ‘What kids?’ ”

The police were calling from the house where the couple lived. Cole suspected it was where the abuse took place. Over the phone, he walked an officer through the house to make sure they were in the right place. The officer spotted a red telescope and black chair in the office and a scratch on the door frame of the master bedroom that were seen in the photos.

Police found the girls four hours later with a family friend.

The couple pleaded guilty to sexually exploiting the girls, to whom they are related, for the production of pornography. Michael Freeman was sentenced to 50 years in prison. His wife got 25 years.

“WE ARE LIKE BROKEN BONES”

The father in the other case is locked up, too. He was sentenced to a minimum of 45 years in state prison for raping his daughter and 45 years in federal prison for filming the attack. The other defendant received 35 years in federal prison for child pornography and a minimum of 31 years in state prison for the rape.

The mother knows her girls are safe from their father now, but every day is a struggle. One daughter tried to commit suicide, another had behavior problems and the youngest still won’t sleep alone in the dark.

“We are like broken bones,” the mother told a judge when one of the men was sentenced three years ago. “People are working to help heal us, but we may never work the same way again.”

Time and therapy have helped, but there is no delete button for the photos on the Internet. She receives a notification from the Justice Department when agents arrest a child pornographer who has an image of one of her daughters on his computer. She has bags full of weekly notifications.

“It’s bittersweet,” she says. “I’m glad somebody else was caught. … But there’s no end to it — ever — as long as there’s a way to disseminate it on the Internet.”