(Reuters) - Two Cambodian solar companies will no longer cooperate with a U.S. trade investigation into allegations they are flooding the market with panels priced below their cost of production, according to documents filed with the Department of Commerce.

Solar Long and Hounen Solar, in separate letters dated Oct. 15, said through a U.S. attorney that they were no longer able to dedicate resources to the antidumping probe.

Their attorney, Craig Lewis of Hogan Lovells, had no further comment.

The latest trade case to roil the U.S. solar market began in April, when several domestic manufacturers asked President Joe Biden's administration to impose tariffs on products from Vietnam, Cambodia, Malaysia and Thailand -- the largest sources of panels in the United States.

Solar Long and Hounen Solar were selected by trade officials in June as so-called mandatory respondents in the investigation because of their large export volumes to the United States. Mandatory respondents serve as proxies for other producers and are eligible for a separate tariff rate than the one Commerce applies country-wide.

Solar imports from Cambodia are down about 37% this year, according to U.S. Census Bureau data.

It was not immediately clear if the companies would continue to participate in the related anti-subsidy investigation.

Commerce Department officials were not immediately available for comment.

An attorney for the companies behind the trade case, which include Hanwha's Qcells and First Solar ( FSLR ), said Commerce would likely penalize the Cambodian producers for withdrawing from the probe.

"This is a significant development, because with both Cambodian respondents withdrawing from the antidumping investigation, they are failing to cooperate with the Commerce Department's investigation," Tim Brightbill, an attorney for the American Alliance for Solar Manufacturing Trade Committee, said in a statement.