Federal authorities have seized the website backpage.com and affiliated websites, according to a notice posted by the FBI and other law enforcement agencies on the site on Friday.
The notice did not detail the reason for the seizure but noted that Justice Department's Child Exploitation and Obscenity Section, as well as the offices of the Texas attorney general and California attorney general, were involved.
Backpage.com, an online classifieds site, advertises everything from cars to furniture.
However, critics have said that the bulk of its revenue comes from its adult-oriented section, which advertises for “escorts” and "sensual massages."
While these services are technically legal, law enforcement and some members of Congress have said they are often thinly-veiled code for prostitution and underage sex trafficking.
Backpage has said such ads are constitutionally protected free speech.
Last year, a report by the Senate Permanent Subcommittee on Investigations alleged that Backpage would strip out terms that would signal underage trafficking such as “Lolita, “Little Girl” and even “Amber Alert” from ads in its "adult" section.
The scrubbed ads would then post to the site, the report said.
Backpage could not be immediately reached for comment on Friday's website seizure but previously told ABC News in a statement that it hires moderators who diligently work to screen ads and curb underage trafficking on its site. They added that they have voluntarily undertaken a multi-tiered "policing system to prohibit and report attempts at human exploitation and the advertisement of prostitution" that screens for words and phrases that might "suggest illegal activity." The company also said it actively cooperates with law enforcement and reports suspicious ads to the National Center for Missing and Exploited Children, or NCMEC.
ABC News' “Nightline” conducted a more than year-long investigation into underage sex trafficking through ads Backpage.com and the efforts to stop it.
Sen. Claire McCaskill, a Missouri Democrat, who has long worked on the issue, applauded the website seizure on Friday.
"This is great news for survivors, advocates, and law enforcement in Missouri and across the country, but it’s also long-overdue, and further proof of why our bipartisan legislation is so critical," she said in a statement and added that law enforcement need a measure to allow them to move quickly against websites that knowingly help facilitate the sex trafficking of minors.
ABC News' Gloria Riviera, Sally Hawkins, Jenna Millman and Jackie Jesko contributed to this report.