Feinstein, D-Calif., interpreted the findings by the Government Accountability Office as supporting the contention Guantanamo can be closed, CNN reported.
"This report demonstrates that if the political will exists, we could finally close Guantanamo without imperiling our national security," Feinstein said Wednesday.
The GAO, however, suggested closing the facility would be complicated.
Most of the Defense Department facilities identified as possible prisons for the Guantanamo detainees are on busy military bases and could be accessible to people wanting to harm the detainees or their keepers.
Fox News said it obtained a leaked copy of the report and Feinstein, chairwoman of the Senate Intelligence Committee, only released it after Fox published information on it. Feinstein requested the GAO report in 2008.
Since then, Congress has barred detainees being transferred to civilian U.S. prisons.
Guantanamo now holds 166 prisoners.
Sen. John McCain of Arizona, the top Republican on the Senate Armed Services Committee, said on Fox News Wednesday he has "great objections" to the GAO report.
He said such a move "would be in violation of legislation that the Congress passed" and would be "basically an assertion of the executive authority that clearly would be violating existing laws."
"I think it would be very hotly contested," he said. "And by the way, an example of the deterioration in Iraq, a guy ... who was responsible for a very sophisticated operation that executed five American soldiers was in prison in Baghdad, he was just released. He went back to Lebanon where he's one of the Hezbollah leaders."