WASHINGTON — U.S. taxpayers are unlikely to recover their $81 billion investment in General Motors Co. and Chrysler Group LLC and were "left in the dark" on specifics of a decision to aid automakers, a congressional panel said.

The Treasury Department should consider placing its GM and Chrysler ownership stakes into an independent trust to prevent "political pressure and government interference," the Congressional Oversight Panel said in a report yesterday.

"Even if no direct conflict exists, a trust could prevent the use or appearance of political influence in the government's ownership," the panel concluded.

The report didn't estimate how much of taxpayers' aid to the auto industry will be recovered. The panel said GM stock would need "highly optimistic" returns for the full investment to be repaid.

The report of the panel, which oversees the Troubled Asset Relief Program, raises questions about the Obama administration's transparency in aiding automakers and challenges the Treasury Department to make more disclosures about company decisions and the government's future role.

"Congress and ultimately the American taxpayer have been left in the dark concerning details of Treasury's review process and its methodology and metrics at a time when Treasury committed additional TARP funds to these companies," the panel said.

"The Treasury auto team failed to disclose to the public both the factors and criteria it used in its viability assessments, the scope of outside involvement in its evaluations, and its basis and reasoning for selecting particular benchmarks," according to the report.

"Simply, its disclosures did not go far enough."