(Reuters) - The Republican-led U.S. House of Representatives Judiciary Committee is seeking information from some 60 U.S. asset managers about their involvement with an investor climate group, adding pressure against environmental efforts by large investors.

Letters sent on Friday to members of the Net Zero Asset Managers initiative, or NZAM, were signed by committee Chairman Jim Jordan and Representative Thomas Massie. They made claims in line with a committee report released on Dec. 13 that Republicans say showed fund firms colluded to cut emissions. The committee's Democrats have dismissed such claims, and big fund firms have denied similar charges.

Representatives for companies that received the letter including BlackRock ( BLK ), State Street and JPMorgan Asset Management did not immediately comment when contacted late on Friday.

Republicans previously have taken credit for prompting those three fund managers to step back from another investor group, the Climate Action 100+.

NZAM says it is an international group with more than 325 signatories managing $57.5 trillion, according to its website. Members pledge to support the goal of net zero greenhouse gas emissions by 2050, using influence such as how they vote their proxies at corporate meetings.

The letters from Jordan and Massie state that companies' efforts with NZAM and the affiliated Glasgow Financial Alliance for Net Zero "may violate U.S. antitrust law," citing the earlier report. They ask for information such as how companies' participation in NZAM changed their stewardship strategies.

Mindy Lubber, CEO of Boston-based environmental advocacy group Ceres, an organizing partner of NZAM, said in an interview that the letters were "consistent with other efforts to suggest that investors ought not to consider climate risk, when of course they should be aware of climate risk as part of their fiduciary duty."