"By April 25, I want to know the status of that directive and [how] it relates to individual task forces and, of equal importance, whether the task forces have been charged with specifically focusing on classified advertising — the latest front in this ever-evolving battle to combat human trafficking," Wolf wrote.

Wolf also spent considerable time detailing what is known about the website Backpage.com, a classified ad index that includes adult services. Wolf said The New York Times has documented how Backpage contains significant advertising for prostitution, and that the National Association of Attorneys General believes this site to be the "premier website for human trafficking in the United States."

Wolf said senators and other groups have pushed Backpage to remove its adult services section, as Craigslist has done, but to no avail.

"And yet, in the face of public pressure and even shame, Backpage.com remains unmoved," Wolf wrote. "I believe it is imperative that these efforts be complemented by the very real prospect of criminal prosecution. Law enforcement, notably the Department of Justice, must engage."

On Tuesday, Reps. Marsha BlackburnMarsha BlackburnButtigieg hits back after parental leave criticism: 'Really strange' Senate approves short-term debt ceiling increase Hillicon Valley — Presented by American Edge Project — Facebook experiences widespread outage MORE (R-Tenn.) and Carolyn Maloney (D-N.Y.) wrote to Google CEO Larry Page to ask a series of questions about what that company is doing to prevent human trafficking. The two members said there are complaints from human-rights groups that Google's advertising practices contribute to trafficking.

Among other things, Blackburn and Maloney asked what internal training steps Google is taking to implement policies combating human trafficking and how it might disclose profits related to ads that support the practice.