0301 GMT December 14, 2018
The fresh scrutiny of Mattis, often portrayed at home and abroad as a trusted steward of US values during the turbulent times of President Donald Trump, came on the heels of his implementation of a controversial military order to place troops on the US-Mexico border, a move critics slammed as a political stunt, AFP reported.
The most vocal attack on the former Marine general came from a member of Trump's own Republican Party this week, when Senator Lindsey Graham blasted the Pentagon chief and secretary of state, Mike Pompeo, for refusing to directly link Saudi Crown Prince Mohammed bin Salman to Khashoggi's murder at the kingdom's Istanbul consulate in October.
Mattis has repeatedly condemned the killing and called for those responsible to be held to account, but insisted he had seen "no smoking gun" connecting bin Salman to the Khashoggi murder.
"You have to be willfully blind" not to conclude the murder was orchestrated by people under prince Mohammed's command, Graham said, following a briefing to several senators by CIA Director Gina Haspel.
"There's not a smoking gun, but a smoking saw," Graham added, referring to the reported grisly detail that an autopsy specialist dismembered Khashoggi's body with a bone saw.
Graham is a firebrand in US politics, and his bouts of indignation should be viewed through the prism of his own ambition. Initially a fierce Trump opponent, he converted to a staunch ally, and Washington observers say he is angling for a top posting in the administration.
'I need the evidence'
Still, Graham was not alone in his upbraiding.
Democratic Senator Chris Murphy said Mattis and Pompeo have tried to "push aside" the question of bin Salman’s involvement and said that when the two men spoke to senators last week they had sought to mislead lawmakers.
They "knew that there was no way this murder happened without the consent and direction of MBS," Murphy told MSNBC, using the abbreviation for Mohammed bin Salman.
And Republican Bob Corker, who leads the Senate Foreign Relations Committee, came to a similar conclusion, saying a jury would convict the bin Salman "in less than 30 minutes."
On Wednesday, Mattis said Graham has "the right to his own opinion" and reiterated his careful interpretation of the intel on Khashoggi's murder.
"If I say something, I need the evidence," Mattis said.
"We are continuing to review. I am quite satisfied we will find more evidence of what happened. I just don't know what it is going to be or who will be implicated, but we will follow it as far as we can."
Saudi Arabia has sought to distance bin Salman from the murder and has received unbending support from Trump, who sees Riyadh as a vital security partner in the Middle East and a key oil exporter and buyer of US arms.
But US lawmakers have grown increasingly leery about American support for the Saudi-led military campaign in Yemen.
A bipartisan group of senators on Wednesday introduced a resolution that, if approved, would say the Senate "has a high level of confidence" bin Salman was "complicit" in Khashoggi's killing, and would assail Riyadh for its role in Yemen's humanitarian crisis.
The Senate could also vote on a separate measure next week to force the US to end its military support to the Saudi-led coalition in Yemen.
Mattis's cautious words come at a sensitive time. He must tread a fine line with Saudi Arabia as he publicly and privately pushes Riyadh to negotiate for a peace settlement with Houthi Ansarullah movement in Yemen.
For his part, Trump has said "maybe he did and maybe he didn't" when asked if bin Salman knew about the plot to kill Khashoggi.
Graham suggested Mattis and Pompeo were being vague in their intelligence assessments to please Trump.
"The reason they don't draw the conclusion that he's complicit is because the administration doesn't want to go down that road, not because there's not evidence to suggest he's complicit," Graham said.