A 3-month-old girl and her 5-year-old sister were rescued in Colorado last week from a child predator, who was offering to sell the children for sex, the FBI said Wednesday.

A friend who was staying with the girls’ family had made a deal with an undercover officer who was part of the FBI Denver’s Rocky Mountain Innocence Lost Task Force, according to a news release from the FBI.

The family friend was offering access to the girls for sexual purposes in exchange for $600, the FBI said. The person, who has not been identified by authorities, was arrested.

The FBI is working with Colorado’s Child Protective Services to conduct a forensic interview and make sure the children are living in a safe environment. The FBI did not release where in Colorado the arrest had taken place.

The arrest and rescue were announced as part of the FBI’s Operation Cross Country XI, a national sting focused on preventing prostitution and human trafficking. In all, 55 FBI field officers and 78 FBI-led task forces participated in stings at hotels, casinos, truck stops and on social media sites used by pimps, prostitutes and their customers.

“The threat of child sex trafficking is something the FBI works on every single day,” Calvin Shivers, special agent in charge of the Denver Division, said in the news release. “Operation Cross Country gives us the opportunity to shine a light on this threat and to educate the public.”

The FBI has not released further details on the person arrested for trying to sell the children for sex.

Denver’s Innocence Lost Task Force covers Colorado and Wyoming. During the operation, 17 minors were rescued, and four people were arrested on suspicion of commercially exploiting children and/or adults, the FBI’s Denver office said in a news release.

Another 27 adults were arrested or cited on suspicion of prostitution along with 10 customers. Eighteen were arrested after they allegedly tried to purchase sex with children, the news release said.

Forty-seven adults who were working in the illegal sex industry were contacted by law enforcement and offered help in leaving the sex trade. The FBI’s victim specialists worked with local victims advocates and nonprofits to provide medical and mental health assistance, food, clothing and shelter.