Mitt Romney passed up on his last best chance to put President Barack Obama on the defensive over his handling of a deadly September 11 attack on the US mission in Libya when the Republican challenger held fire on the issue in their final debate on Monday.

Obama came to the showdown in Boca Raton, clearly prepared for Round Two over the events surrounding last month's killings of the U.S. ambassador and three other Americans at the consulate in Benghazi, a big vulnerability for the president on foreign policy just two weeks before the November 6 election.

But Romney, whose misstep on Libya in their previous debate may have left him feeling singed, shied away from a fight - a decision that could disappoint many conservatives who hoped to see their candidate on the attack.

Obama was the only one of the two candidates even to utter the word "Benghazi" during the 90-minute debate focused on foreign policy, and the president even said later at one point, "Going back to Libya..." when another question was asked.

Asked why Romney had pulled his punches on the issue, Romney senior adviser Eric Fehrnstrom said, "He'd addressed it plenty of times in other forums and there were many other areas to cover."

But US Democratic Representative Debbie Wasserman Schultz, acting as one of Obama's surrogates at the debate site at Lynn University, said Romney had little choice but to soft-pedal his criticism of Obama on the issue.

"It's no surprise," she told Reuters. "He's been spanked repeatedly for politicizing a tragedy."

But Republicans are sure to ask whether Romney missed another opportunity.

In the most vividly testy exchange of last week's debate, Romney said Obama took weeks to acknowledge that the Benghazi assault was a terrorist attack.

Obama, who spoke of "acts of terror" in an appearance in the White House Rose Garden the day after the attack, challenged Romney to "check the transcript" and chastised him for trying to score political points on the deaths of Americans overseas.

The moderator of that debate, CNN's Candy Crowley, backed Obama's interpretation of events, saying, "He did call it an act of terror."

In fact, some of Obama's top aides initially attributed the Benghazi violence to protests over an anti-Islam film and said it was not premeditated, before acknowledging much later that it was a terrorist attack.

But a slew of pundits dubbed it Romney's "Libya moment," and some of his own aides conceded privately that Obama got the better of him.

Moderator Bob Schieffer gave Romney a wide-open chance for another swing at the issue on Monday when he noted that questions remained that the administration had yet to answer about the incident.

"What happened? What caused it? Was it spontaneous? Was it an intelligence failure? Was it a policy failure? Was there an attempt to mislead people about what really happened?" Schieffer asked.

Romney chose to tread softly.

"We see in Libya an attack apparently by, I think we know now, by terrorists of some kind against - against our people there, four people dead," he said. "Our hearts and minds go out to them."

He moved from that to criticism of Obama's overall policy toward the Arab Spring revolts, but never really drew a strict connection.

Obama managed to dodge the more pointed parts of Schieffer's question, including shifting explanations of what happened and whether the administration had failed to provide requested security for the consulate.

"When we received that phone call, I immediately made sure that, number one, that we did everything we could to secure those Americans who were still in harm's way; number two, that we would investigate exactly what happened, and number three, most importantly, that we would go after those who killed Americans and we would bring them to justice," Obama said.

"And that's exactly what we're going to do."

Despite that, Fehrnstrom said, "There are many more questions than there are answers on this from the administration."