After tricking several thousand executives into downloading malicious software earlier this week, online scammers started up their subpoena phishing scam again Wednesday, but on a much smaller scale.

First reported Monday, the phishers send a small number of e-mail messages to senior executives within companies, often CEOs, telling them that they've been subpoenaed for a federal court case. The e-mails direct the victim to a Web site that is very similar to a legitimate California federal court page, but ending in "...-uscourts.com," rather than the "....uscourts.gov" Web domain actually used by federal courts.

Although they end with the same letters, the domains used in this scam are actually different from and not connected with the uscourts.com Web site, which offers access to court records in some jurisdictions.

The e-mail sent to executives is specially crafted to appear legitimate, a tactic called "spear-phishing." The emails include the executive's name, company's name and even the correct phone number.

Executives who click on the link in the e-mail are then told that they need to download a plug-in in order to read the subpoena. That plug-in is actually malicious software.

Although the U.S. federal court system uses email to communicate information about cases, subpoenas for new cases are not served via e-mail.

Verisign, which estimates that about 2,000 people were tricked by the scam on Monday, believes that Wednesday's attack was on a much smaller scale. Late Wednesday the company's iDefense group had tracked only about 100 infections, said Matt Richard, director of iDefense's Rapid Response Team.

Security experts have been fighting the phishers. By Tuesday they'd managed to get the first phishing Web site taken down, only to have the second one pop up on Wednesday.

Because the attack targets such a small number of victims, anti-spam companies have had a hard time filtering the e-mails and antivirus companies have been similarly pressed to block the malicious software that the attackers are using.

Late Wednesday, antivirus companies were not blocking this latest version of the malware, said John Bambenek, a security researcher at the University of Illinois at Urbana-Champaign and volunteer at the SANS Internet Storm Center.