Senators Patrick Leahy and Bob Casey, prominent voices on foreign policy in President Barack Obama's Democratic Party, said the island had failed to carry out recommendations from its own Lessons Learnt and Reconciliation Commission (LLRC).
"The Sri Lankan people deserve better. In fact, as the government of Sri Lanka fails to implement LLRC recommendations, the outlook for human and political rights in Sri Lanka appears to be getting worse," the senators wrote in a letter to outgoing Secretary of State Hillary Clinton.
The senators voiced concern over the recent impeachment, backed by President Mahinda Rajapakse, of the chief justice and highlighted charges of human rights groups that authorities have intimidated the media.
"Accountability is a necessary precursor to reconciliation and a stable democracy in Sri Lanka," the senators wrote.
"It is clear to us that the LLRC process is mired in bias and delays, and only an independent, international investigation will achieve real account