"While there is no silver bullet to end sex trafficking, to stop its largest beneficiary is a huge step in the right direction", Krell said following the hearing.
The indictment, unsealed Monday, charges that personal ads on the Backpage.com site allowed prostitution and sex trafficking, netting more than $500 million since the site's creation in 2004.
Ferrer additionally pleaded guilty to state money laundering charges in California as well as Texas, where the company itself pleaded guilty to human trafficking, according to the attorneys general in those states.
Four employees and the site's founders pleaded not guilty to the federal charges.
The chief executive of a Dallas-based website, who pleaded guilty to one count of conspiracy and three counts of money laundering, will only serve maximum of five years in prison under a California agreement.
The 61-page indictment, returned under seal March 28 by a federal grand jury, claimed that "virtually every dollar flowing into Backpage's coffers represents proceeds of illegal activity".
Ferrer also agreed to cooperate in the ongoing California prosecution of Backpage.com founders Michael Lacey (left), and James Larkin (right).
Backpage.com and its affiliated websites were seized on Friday by US federal law enforcement authorities and taken off the internet.
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And a federal judge in Phoenix unsealed an April 5 plea deal revealing that Ferrer pleaded guilty to conspiracy, and Backpage.com pleaded guilty to money laundering conspiracy. California prosecutors announced earlier Thursday that Ferrer had pleaded guilty to money laundering in that state.
Becerra also called the latest legal victories against Backpage "a game-changer in combatting human trafficking".
Maggy Krell, a former assistant attorney general in California and lawyer who worked on the case, says that the recent pleas and indictments could be a major turning point. This comes less than a week after the attorney general's office assisted the Department of Justice with permanently shutting down the website.
State agents raided the Dallas headquarters of Backpage and arrested Ferrer on a California warrant after he arrived at Houston's Bush Intercontinental Airport on a flight from Amsterdam on October 6, 2016. Backpage was owned by a Dutch-based company.
But Brown threw out pimping conspiracy and other state charges against Backpage's operators.
SESTA aims to halt sex trafficking, particularly of children, by restricting what kind of information can be posted on websites like Backpage, where people often advertise sexual services.
The site was the dominant hub for sex workers to advertise their services, and it had come under increasing scrutiny in recent years as the U.S. government made efforts to crack down on sex trafficking with legislation like the Stop Enabling Sex Trafficking Act (SESTA), which President Trump signed this week.