NEW YORK (Reuters) - A bipartisan group of 13 U.S. senators told the U.S. Environmental Protection Agency on Tuesday to disclose the names of companies who received a hardship exemption from the nation’s biofuel laws since 2016.

The calls for more transparency come amid increasing scrutiny of the EPA’s expansion of the small refinery hardship exemption in recent months, including granting and considering waivers from the world’s largest oil companies.

The senators made the request in a letter delivered on Tuesday, according to a joint statement.