Some of the sites involved were seized from net entrepreneur Kim Dotcom and are thought to attract large numbers of visitors.
The sites were taken over thanks to an administrative error by the FBI which forgot to renew its ownership of one domain.
The sites have now been suspended and no longer show any web content.
Tech news site Torrent Freak revealed that several web domains that used to be run by Kim Dotcom were suddenly directing visitors to sites peddling porn, fake security software, adware and bogus special offers.
These domains were seized three years ago when the FBI carried out raids on Mr Dotcom's business empire, alleging that his Megaupload service was a haven for pirates and copyright thieves. The legal action that started with the raids has yet to be resolved.
Megaupload.com and Megavideo.com were two of the domains briefly taken over by the adware peddlers.
Mr Dotcom lamented what happened to the domains in a tweet, saying the US government had "lost control" of them. He also warned people about adware and other bogus offers.
A separate investigation by news site Ars Technica revealed that the domains had become available because the FBI had forgotten to renew its ownership of a separate site - cirfu.net.
This was used by the agency's Cyber Initiative and Resources Fusion Unit that oversaw sites seized during investigations and raids conducted by the force. This unit replaced the seized site's content with banners announcing that they had been taken over by the FBI.
Megaupload.com, an associated Mega video site and several gambling domains were controlled via cirfu.net.
However, said Ars Technica, cirfu.net was auctioned earlier this month soon after the FBI's ownership of it expired and was not renewed.
The exact identity of who got control of cirfu.net and its associated sites is not known. However, net ownership records show it is now run by Syndk8 Media which is based in Gibraltar.
Soon after news stories about the sites began circulating, the sites were suspended by registrar GoDaddy for violating the company's terms and conditions.
The FBI has yet to comment on what happened to the domains.
Websites overseen by the FBI have been briefly used to offer visitors porn or links seeded with adware.