The Federal Trade Commission won a preliminary legal victory against what it called one of the largest spam gangs on the Internet, persuading a federal court in Chicago on Tuesday to freeze the group’s assets and order the spam network to shut down.

The group, which used several names but was known among spam-fighting organizations as HerbalKing, sent billions of unsolicited messages to Internet users over the last 20 months, promoting replica watches and a variety of pharmaceuticals, including weight-loss drugs and herbal pills that supposedly enhanced the male anatomy, according to the commission.

“This is pretty major. At one point these guys delivered up to one-third of all spam,” said Richard Cox, chief information officer at SpamHaus, a nonprofit antispam research group.

The investigation provides a clear window into the business of modern spam, which by some estimates accounts for 90 percent of all e-mail sent over the Internet.