Oct 21 (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.