Attorneys general in 45 states are asking a classified ads website to explain how it handles postings for adult services.
The officials said in a letter to Backpage.com on Wednesday that hundreds of ads on the site are for prostitution, and the site attracts people seeking to exploit minors.
They want Backpage.com to prove it is monitoring the site to prevent illegal activity and to willingly provide information in lieu of a subpoena.
Carl Ferrer, a spokesman for Backpage owner Village Voice Media, did not immediately respond to a message seeking comment.
Craigslist closed its adult services section last year after attorneys general and others raised concerns it could not effectively screen out illegal ads.