WASHINGTON -- A seasoned diplomat who penned a highly critical review on security at the U.S. consulate in Benghazi, Libya, defended his report that blamed State Department deficiencies but absolved then-Secretary of State Hillary Rodham Clinton.

Thomas Pickering, who has served in Republican as well as Democratic administrations over a career that spanned four decades, stood by his panel's conclusion that decisions about the consulate were made well below the secretary's level.

His comments during several television show appearances Sunday failed to placate Republicans who have called for a special select congressional committee to investigate the Sept. 11, 2012, assault on the diplomatic mission that killed four Americans, including U.S. Ambassador Chris Stevens.

Democrats say Republicans are trying to exploit the Benghazi deaths to undercut Clinton, an early favourite for the Democratic presidential nomination in 2016.

"We knew where the responsibility rested," said Pickering, who headed the Accountability and Review Board that investigated the attack, along with retired Adm. Mike Mullen, the former chairman of the Joint Chiefs of Staff.

"They've tried to point a finger at people more senior than where we found the decisions were made," Pickering said of Clinton's critics.

Pickering and Mullen's report released in December found that "systematic failures and leadership and management deficiencies at senior levels" of the State Department meant that security was "inadequate for Benghazi and grossly inadequate to deal with the attack that took place."

The Obama administration has tried to move past the controversy, but a steady drip of new information is fueling Republican claims that the government initially misled the public about the nature of the assault.

The House Oversight and Government Reform Committee last week heard a riveting minute-by-minute account from a former top diplomat in Libya about the two nighttime attacks. Gregory Hicks, a former deputy chief of mission to Libya, detailed his phone conversations from Tripoli with Stevens.

Hicks and two other State Department witnesses criticized the Pickering and Mullen's review. Their complaints centred on a report they consider incomplete, with individuals who weren't interviewed and a focus on the assistant secretary level and lower.

The hearing produced no major revelation but renewed interest in the attacks that happened during the lead-up to the November 2012 presidential election.

The top Republican on the oversight committee, Rep. Darrell Issa, said he wants sworn depositions with Pickering and Mullen. Issa said his panel has not been provided sufficient details on the State Department review, such as a list of everyone the investigators interviewed or a full transcript of those conversations.

"We want the facts. We're entitled to the facts. The American people were effectively lied to for a period of about a month," Issa said.

Republicans continue to explore incident

Republicans are insisting on exploring what happened at the consulate, what might be done to prevent future such attacks and what political calculations went into rewriting talking points the U.S. Ambassador to the United Nations, Susan Rice, used on news shows the Sunday after the attack.

A series of emails that circulated between the State Department and the CIA led to weakened -- and, in some cases, wrong -- language that Rice used to describe the assault during a series of five television interviews the Sunday after the attacks.

"I'd call it a coverup," said Sen. John McCain, a Republican. "I would call it a coverup in the extent that there was wilful removal of information, which was obvious."

"I was surprised today that they did not probe Secretary Clinton in detail," Sen. Kelly Ayotte said, of the review board.

One Republican eyeing a White House run, Sen. Rand Paul, said at a public appearance that he thinks the Benghazi attack "precludes Hillary Clinton from ever holding office."

Clinton's allies said Republicans were looking to weaken her ahead of a potential 2016 campaign.

"This has been caught up in the 2016 presidential campaign, this effort to go after Hillary Clinton," said Sen. Dick Durbin, a Democrat. "They want to bring her in because they think it's a good political show and I think that's unfortunate."

Pickering spoke with CNN's "State of the Union," NBC's "Meet the Press" and CBS' "Face the Nation." Issa and Feinstein spoke with NBC. McCain spoke to ABC's "This Week." Ayotte and Durbin were on CBS. Smith spoke to "Fox News Sunday."