"Why not have a nonpartisan or bipartisan commission do it, like we did in 9/11, and just go back and find everything that happened?" said the Democratic senator, referring to the investigative panel assembled in the aftermath of the September 11, 2001 terror attacks.
The bipartisan September 11 commission probed the causes that led to the terror attacks on New York and Washington and offered guidelines to prevent future attacks.
Leahy said he was not motivated to propose a similar panel by "some idea of vengeance" but by a quest for the truth.
"We know that there were a number of people that made the decision to violate the law, a number of people who said that we don't have to follow our constitution," he said.
Others who backed harsh treatment for detainees "wrote memos basically saying the president and the vice president are above the law -- that the laws of the United States don't apply to them like they do to you and me," added Leahy.
"I want to know why they did that ... What kind of pressures brought them to write things that are so off the wall, and to make sure it never happens again."
But John McCain, a senior Republican senator with a long track record of opposing torture, argued that establishing an inquiry could prove "divisive" for a country already polarized over the issue.
"It is well known what happened. There (are) going to be pictures that are going to be coming out, which will again authenticate that wrong things were done," said the 2008 Republican presidential nominee.
"But are you going to prosecute people for giving bad legal advice? Are you going to keep on down this road in order?" said McCain, who lost to Barack Obama in his quest for the White House last year.
"We need to put this behind us. We need to move forward," he added. "We need a united nation, not a divided one."
Debate has raged on whether to prosecute officials who backed harsh interrogation for terror inmates, after the Obama administration last week released four sensitive memos that blew the lid on the practice.
The harsh interrogation techniques were approved under former president George W. Bush, and included the use of insects, simulated drowning and sleep deprivation.
Obama has said that CIA officers involved in interrogations should not be prosecuted because they were carrying out orders. But he left the door open to possible prosecution of senior officials in the previous administration.
Senate Intelligence Committee chairwoman Dianne Feinstein called for her panel to be given time to properly examine the cases.
"My hope is that the public debate quells, that we have an opportunity to do our work," she told CNN.
"It is our responsibility to do oversight. We have access to the classified information.
"We have set upon a course, a bipartisan course with a program scope, approved by the committee, to review the conditions of detention and the techniques of interrogation of each of the high value detainees."
She said that probe would likely take "six to eight months," before it could release findings and recommendations.
--------------------------------------------------------------------------------------------------