Green Century Capital Management said on Thursday it has left a high-profile financial industry effort to cut carbon emissions, citing compliance challenges.

Leslie Samuelrich, president of the firm known for its fossil-free funds confirmed that the fund manager had withdrawn from the United Nations-backed Net Zero Asset Managers (NZAM) initiative.

The group was launched in 2020 to encourage fund firms to reach net zero emission targets and limit the rise in global temperatures. But it and similar efforts have given rise to antitrust concerns including from U.S. Republican politicians. Vanguard Group left NZAM in December, citing a need for independence.

Samuelrich said Green Century’s situation was different and somewhat unique, because unlike Vanguard and other major fund firms it does not own oil or coal stocks, making it harder for her firm to fulfill NZAM’s pledges.

While she supports NZAM’s efforts, she said, “There are challenges with compliance for us and I can’t ignore that.”