WASHINGTON -- Commander James Mattis, who just retired from the helm of United States Central Command, told a security conference in Colorado on Saturday he was stumped by the US response to the attempted assassination of Saudi Arabia's ambassador to the Washington by the Iranian government.

Iran allegedly planned on killing Ambassador Adel Al-Jubeir as he dined in Georgetown. The US says the plot was coordinated by Quds forces and well-compensated Mexican drug cartel members.

"Frankly, I‘m not sure why, again, they haven’t been held to account," Mattis told Wolf Blitzer at the Aspen Security Forum. "They have been basically not held to account....I don’t know why the attempt on Adel wasn’t dealt with more strongly."

Mattis said he was confused why US Attorney General Eric Holder announced the plot publicly, saying it undermined the gravity of the allegations and buried the story in legal jargon.

""They actually set out to do it," he emphasized. "It was not a rogue agent off on his own. This decision was taken at the very highest levels in Tehran. Again absent one mistake, they would have murdered Adel and Americans at that restaurant a couple miles from the White House."

Mattis said it is important the Obama administration make clear there are actions the United States will not tolerate. He also cast doubt on the West's ability to physically stop Iran's nuclear program through military strikes.

"I don’t think anyone can destroy a program that has spread out from Tehran to the mountains, from underground facilities to above ground," he said. And much of it is destroying access to underground facilities."

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