The U.S. Securities and Exchange Commission has filed securities fraud charges against two Texas men who allegedly hijacked computers nationwide to send millions of spam e-mails and cheat investors out of more than $4.6 million.

The men used networks of compromised computers, often called botnets, to send out spam about at least 13 penny stock companies, the SEC said Monday. The SEC began to investigate the operation after one of its enforcement attorneys received one of the e-mail messages at work.

Darrel Uselton and his uncle, Jack Uselton, both repeat securities law violators, illegally profited during a 20-month scalping scam, the SEC said. The two obtained shares from 13 or more penny stock companies and sold those shares after pumping up the market through manipulative trading, spam e-mail campaigns, direct mailers and Internet-based promotions, the SEC said.

The SEC defines scalping as recommending that other people purchase a security while secretly selling the same stock.

In related actions, the Attorney General's Office for Texas and the Harris County District Attorney's Office indicted the Useltons for engaging in organized criminal activity and money laundering. The Texas authorities have seized more than $4.2 million from bank accounts controlled by the Useltons, the SEC said.

The SEC's action is "intended to protect investors from fraud artists who would treat the investing public as their personal ATM machines," SEC Chairman Christopher Cox said in a statement. The scheme, by combining get-rich-quick promises, computer intrusions and spam, was a "virulent threat to ordinary investors," he added.

The SEC's complaint, filed in the U.S. District Court for the Southern District of Texas in Houston, alleges that the Useltons used an array of computer botnets to anonymously flood the inboxes of American investors with millions of spam e-mails touting near-worthless penny stocks. Each campaign, which featured a single company, lasted from several days to several weeks.

Between May 2005 and December 2006, the Useltons obtained more than $4.6 million through their fraudulent scheme, the SEC alleged. The Useltons and the companies they controlled typically received unrestricted shares from penny stock companies for little or no money, in return for purported financing or promotional activities, the SEC said.

Darrel Uselton was disciplined by the National Association of Securities Dealers in 2004 and 2005. Jack Uselton was permanently barred from violating the SEC's antifraud provision in a 2002 settled action, the SEC said.

The SEC in March 2007 suspended trading in the securities of three of the companies involved as part of its antispam initiative. The SEC revoked the registration of a fourth company in December 2005.