Obama's statement came amid a spirited back-and-forth with Romney over the assault on the consulate, the only significant foreign-policy disagreement in an hour-and-a-half exchange dominated by domestic concerns.
Romney challenged the president to explain why U.S officials argued for more than a week after the Sept. 11 assault that it stemmed from a protest against an American-made film ridiculing Islam.
"Whether there was some misleading, or instead whether we just didn't know what happened, you have to ask yourself why didn't we know," Romney said.
"It was very clear this was not a demonstration," he said. "This was an attack by terrorists."
The two also traded jabs on how quickly the president declared Benghazi an act of terror - with Romney insisting it took two weeks and Obama saying he said as much the day after in an address from the White House Rose Garden. That drew an intervention from the moderator, CNN's Candy Crowley, who appeared to side with Obama.
Before the debate, Clinton tried her best to defuse the issue.
"I take responsibility," she said, reiterating comments she made in a television interview late Monday. "I'm in charge of the State Department's 60,000-plus people all over the world, 275 posts."
Republican senators sought to shine the spotlight back on Obama, crediting Clinton for "a laudable gesture" while insisting that responsibility for the Benghazi attack lies squarely on the president.
"I think it's very laudable that she should throw herself under the bus," Sen. John McCain (R., Ariz.) told Fox News on Tuesday. "But first of all, responsibility for American security doesn't lie with the secretary of state. It lies with the president of the United States."
Clinton rejected that the postattack explanations were intentionally misleading. "Everyone who spoke tried to give the information they had," she said.