A group of House Democrats is calling on Republican leaders to investigate a prominent Washington law firm and three federal technology contractors, who have been shown in hacked e-mails discussing a "disinformation campaign" against foes of the U.S. Chamber of Commerce.

In a letter to be released Tuesday, Rep. Hank Johnson (D-Ga.) and more than a dozen other lawmakers wrote that the e-mails appear "to reveal a conspiracy to use subversive techniques to target Chamber critics," including "possible illegal actions against citizens engaged in free speech."

The lawmakers say it is "deeply troubling" that "tactics developed for use against terrorists may have been unleashed against American citizens."

The call for a congressional probe marks the latest development in the controversy over tens of thousands of e-mails stolen from HBGary Federal, whose computer system was attacked in early February by members of a loose collective of unidentified hackers known as Anonymous.

The e-mails, which are widely available on file-sharing sites, show HBGary Federal, Berico Technologies and Palantir Technologies teaming up with a sales pitch to undermine chamber opponents.

The companies proposed forming a "corporate information reconnaissance cell" and discussed tactics such as creating online personas to infiltrate activist Web sites; planting false information to embarrass U.S. Chamber Watch and other groups; and trolling for personal information using powerful computer software.

The e-mails contain test runs in which the firms culled personal information, including family and religious data, on anti-chamber activists.

The chamber has denied knowledge of the proposals.

The three security firms named in the e-mails have substantial federal contracts. A sales document produced for the Hunton & Williams law firm in November said the firms have "extensive experience in providing game-changing results across the Intelligence Community and defense/government sector."

Other e-mails contain similar proposals to target supporters of WikiLeaks on behalf of Bank of America, which fears it will be that group's next target. Bank of America has denied knowledge of the proposals.

HBGary Federal chief executive Aaron Barr, whose voluminous and voluble e-mails were at the center of the controversy, announced his resignation Monday. Berico and Palantir have condemned the proposals and severed ties with HBGary Federal. Hunton & Williams, the law and lobbying firm that negotiated with the tech companies, has declined to comment.

The Anonymous hacking collective launched its latest apparent attack over the weekend against Americans for Prosperity, a conservative group with ties to the tea party movement. The group's Web site was knocked out of service for extended periods Sunday as a result of the attacks.

An Internet statement by a purported Anonymous member Sunday said AFP was targeted because of its ties to Charles and David Koch, the billionaire brothers whose contributions to conservative causes and politicians have become a flashpoint in the ongoing labor dispute in Wisconsin.

Several corporate Web sites connected to the Kochs experienced problems Sunday, according to media reports.

AFP President Tim Phillips decried the "illegal attack on our free speech rights" in a statement Monday.

"Americans for Prosperity will not be intimidated and will not be deterred," he said.