Washington (CNN) A Friday exchange between Democratic presidential candidate Joe Biden and a Fox News journalist has drawn attention to Biden's shifting accounts of what he advised President Barack Obama about the 2011 raid that killed Osama bin Laden.

Doocy asked Biden if he would be willing, as commander-in-chief, to thwart an "imminent attack on Americans" by using an airstrike to kill a "terrorist leader." (Trump claimed on Friday, without providing evidence, that Soleimani was planning a "very major attack.")

"Well, we did," said Biden, who served as vice president to Obama. "The guy's name was Osama bin Laden."

Doocy responded, "Didn't you tell President Obama not to go after bin Laden that day?"

To which Biden responded: "No, I didn't. I didn't."

Biden's description of his advice to Obama changed between 2012 and 2015; two of his accounts were contradictory. He said in 2012 that he had advised Obama "don't go" with the raid before obtaining more information. He then said in 2015 that he had not uttered this "don't go" opinion.

Former top officials in the Obama administration have written in their memoirs that Biden was "against the operation," that he was "firmly in favor of waiting for more information," and that he was concerned about the risks of a raid.

What Biden said initially

At a Democratic congressional retreat in 2012 , Biden told party legislators that he had advised Obama at the April 2011 meeting not to proceed with the raid before they could be more certain that bin Laden was present at the compound in Abbottabad, Pakistan, where he would be found and killed.

Biden's apparent purpose in telling the story was to vouch for Obama's "backbone" in authorizing the May 2011 raid over the uncertainty of advisers, most of whom Biden said gave tentative answers when Obama went around the table asking for their opinions.

"(When he) got to me, he said, 'Joe, what do you think?' And I said, 'You know, I didn't know we had so many economists around the table.' I said, 'We owe the man a direct answer. Mr. President, my suggestion is don't go. We have to do two more things to see if he's there,'" Biden said.

Biden did not specify what the "two more things" were.

Jay Carney, then the White House press secretary, told CNN's Jake Tapper, then reporting for ABC, that Biden was "speaking accurately" at the retreat about what had happened at the 2011 meeting.

What Biden said later

On NBC's "Meet the Press" in May 2012, Biden added new detail to the story.

While not denying host David Gregory's assertion that Biden had advised Obama "don't do it, don't do it now," Biden said he had an additional conversation with Obama as they walked privately together after the meeting -- during which he said he told Obama, "Follow your instincts, Mr. President. Your instincts have been close to unerring. Follow your instincts." He added, "I wanted him to take one more day to do one more test to see if he [bin Laden] was there."

That version of the story was not incompatible with his initial account to the Democratic legislators. But a third version of the story, which he offered in October 2015 , was incompatible with that initial account.

This time, Biden said that he did not give Obama a "don't go" opinion at the 2011 meeting. (He said he had said merely that they should make "one more pass" with a surveillance drone to make sure bin Laden was present.) Rather, he said , he withheld his opinion until he was alone with Obama after the meeting -- then made clear to Obama, "as we walked out of the room, and walked upstairs," that "I thought he should go."

"Imagine if I had said in front of everyone, 'Don't go' or 'Go,' and his decision was a different decision; it undercuts that relationship. So, I never, on a difficult issue, never say what I think, finally, until I go up in the Oval with him alone," Biden said at a George Washington University panel discussion with former vice president Walter Mondale.

In other words: Biden said in 2012 that he had believed he owed Obama a firm opinion at the group meeting, and had said "don't go" in that setting. Biden said in 2015 that "it would have been a mistake" to offer a firm opinion at the group meeting, so he had never said either "don't go" or "go" in that setting -- but, instead, had privately advised Obama to "go."

What others have said

As Trump's campaign pointed out on Friday, former Obama officials have written in their memoirs that Biden expressed skepticism or outright opposition to the raid when the security team met.

That does not itself contradict Biden's claim that he expressed support, or something less than opposition, in a one-on-one conversation with Obama. But it does at least raise questions.

Leon Panetta, who was as Obama's Central Intelligence Agency director at the time, wrote in his 2014 memoir, Worthy Fights, that Biden argued at the meeting "that we still did not have enough confidence that bin Laden was in the compound, and he came out firmly in favor of waiting for more information."

Robert Gates, who was secretary of defense at the time of the raid, wrote in his 2014 memoir, Duty, that "Biden was against the operation" when Obama went around the table. Gates, who was highly critical of Biden in the book, wrote that the two of them were the "two primary skeptics" of the raid; he wrote , "Biden's primary concern was the political consequences of failure."

Hillary Clinton, who was secretary of state at the time, wrote in her 2014 memoir, Hard Choices, that Biden "remained skeptical" and, like Gates, was concerned about "the risks of a raid."

CNN national security analyst Peter Bergen, author of the book Manhunt about the search for bin Laden, interviewed many of the attendees of the April 2011 meeting in the months after the raid. Bergen reported that Biden expressed concerns at the meeting about the possible Pakistani reaction to a raid in their country, arguing that they needed to become more sure bin Laden was present before risking a major breach in a critical bilateral relationship.

Bergen said in an email on Saturday: "Now is it possible that Biden spoke to Obama privately later and told him something different? Sure. But it's striking to me, that as far as I know, Obama has never come forward to say that Biden advised him to carry out the raid."

In a 2012 presidential debate with Republican Mitt Romney, Obama made reference to Biden's concerns about conducting a raid in Pakistan.

On Saturday, a Biden campaign official who spoke on condition of anonymity said Biden was never opposed to a raid but just thought more certainty was needed about bin Laden's presence before proceeding at that moment.

"At the outset, the Vice President advised that we first obtain additional confirmation that Bin Laden was indeed at the compound -- but he did not say that the operation shouldn't ultimately go forward. Later, in a one-on-one setting, the VP urged President Obama to 'follow [his] instincts," the official said.