"We strongly believe that any conclusions reached by the Sri Lankan government's commissions must be verified by a parallel international mechanism" backed by the United Nations, the senators wrote.
"Without a means of verification, any findings will lack credibility and true accountability," said the letter, led by Democratic Senator Sherrod Brown of Ohio and Republican Senator Richard Burr of North Carolina.
The senators said that accountability was essential to ensuring reconciliation and warned that history is "littered with failed attempts to bring peace."
"We do not want Sri Lanka to be another one of those failed steps," they wrote.
In a separate letter, the House members urged the United States to use its presidency of the UN Security Council this month to push forward "a credible and effective process for achieving accountability" in Sri Lanka.
The House letter was led by Representatives Danny Davis and Jan Schakowsky, both Democrats from the Chicago area that has a sizable Tamil community.
Sri Lanka last year killed the top leadership of the Tamil Tigers, ending their nearly four-decade campaign for a separate homeland in which the rebels were known for their devastating suicide bombings.
The United Nations has raised concerns about human rights toward the end of the war. The world body has reported that at least 7,000 ethnic Tamil civilians were killed in the final months of fighting.
Sri Lanka's government has strongly rejected any UN or independent probe of war crime allegations and says no civilians were killed in the battle against the rebels.President Barack Obama's administration has urged Sri Lanka to ensure accountability and to promote reconciliation but has stopped short of supporting an international probe.