Craigslist, one of the most popular Web sites in the United States, is on track to increase its revenue 22 percent this year, largely from its controversial sex advertisements. That financial success is reviving scrutiny from law-enforcement officials who say the ads are still being used for illegal ends.

The ads, many of which blatantly advertise prostitution, are expected to bring $36 million this year, according to a new projection of Craigslist’s income. That is three times the revenue in last year’s projection.

Law-enforcement officials have been fighting a mostly losing battle to get Craigslist to rein in the sex ads. At the same time, officials of organizations that oppose human trafficking say the site remains the biggest online hub for selling women against their will.

Last week, in the latest example, the Federal Bureau of Investigation arrested 14 members of the Gambino crime family on charges of, among other things, selling the sexual services of girls ages 15 to 19 on Craigslist.