(CNN) Two sisters, one 5 years old and the other 3 months old, were rescued in Denver by undercover agents during this year's FBI-led sting operation against sex traffickers, officials said.

The alleged trafficker, a friend of the children's family, made a deal to sell them for sex for $600, the FBI said. The friend communicated with an undercover agent, the FBI said in announcing details of Operation Cross Country XI.

While this alleged crime alone is disturbing, 82 other juveniles were rescued during last week's nationwide sweep. Some 120 people were arrested.

Despite the arrest numbers, the work is far from over.

"Unfortunately, the number of traffickers arrested -- and the number of children recovered -- reinforces why we need to continue to do this important work," FBI Director Christopher Wray said in a statement.

For these young children, life may never return to the way it was before. Holly Gibbs, a child sex trafficking survivor and the director of the Dignity Health Human Trafficking Response (HTR) Program, said, "Human trafficking is a global public health crisis. What happened to these children is a tragedy, and they have a long journey ahead of them. But for the moment, I am glad that they are safe."

Average age of victims is 15

On October 13, the second day of the operation, a minor was rescued by the FBI in El Paso, Texas, when a 16-year-old female victim was advertised online for "entertainment," officials said.

The girl was accompanied by a 21-year-old female who offered an undercover agent sexual intercourse with both her and the underage victim for $200, officials said. According to the FBI report, the woman as well as the driver, another female, who took the minor and 21-year-old to the undercover officer's location, were arrested.

Among the recovered victims across the country was one from Russia. Local and state agencies were involved in the operation, as were police as far away as the Philippines and Thailand.

The average age of the victims recovered from this year's operation is 15, the FBI said. Sellers tried to pimp them to outsiders from hotels, truck stops or online.

Sometimes, sex trafficking can involve family members or family friends.

Bradley Myles, CEO of the Polaris Project, which launched a human trafficking hotline, says traffickers often prey on those with trauma in their childhood history. And, he said, there often are problems down the road.

"There are ACEs, adverse childhood experiences, and the more ACEs that someone has the higher likelihood that they'll experience other forms of trauma and abuse in life," said Myles, who was not involved in the operation but said he hopes the children are given the resources they need to someday recover.

Focusing on the victims

At a press conference in Denver on Thursday, District Attorney George Brauchler reminded traffickers that undercover agents and task force officers are always lurking, be it behind cameras, on the Internet or in a casino, prepared to make another bust.

"To those out there who are watching this who might avail themselves at the opportunity to exploit children or to engage in even the adult sex trade, this is a bit of a warning. ... The people that you can't see behind those cameras, they are fins. They are fins in the water and they represent sharks who are there all the time."

"And my advice to you is to stay out of those waters where the kids are, to stay out of those waters where exploited adults are for sex, because they are out there and they will get you. And when they get you, my office will handle the rest."

The FBI continues to work with agencies and organization specializing in child protective services for the 84 rescued children.

There are still thousands of children locked in shackles as chess pieces in a game of sex slavery, and without consistent efforts, Myles said he doesn't think it will end

"We are seeing about an average of 23 cases a day. So we need that daily drumbeat of a 24-hour response throughout the country," he said. "Operation Cross Country doesn't happen every day. But these cases are breaking across our country every day."