"I’ve known her for 40 years, I’m a friend of hers," Joe Biden said as he vouched for Hillary Clinton. | AP Photo Biden defends Clinton on email use

For Vice President Joe Biden, it is "hard to believe" that Hillary Clinton "would do anything intentionally wrong" with respect to the ongoing FBI investigation into her use of a personal email server in an official capacity as secretary of state.

Biden told NPR's "Morning Edition" in an interview taped Thursday and airing Sunday that he could not comment on whether he thinks his former Senate and White House colleague did anything wrong, with the prospect of a potential indictment still looming. The vice president, who endorsed Clinton in June after President Barack Obama and will campaign with her in his hometown of Scranton, Pennsylvania, on Friday, reasoned that if he were to comment, it would create an unwelcome appearance.

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"If I comment and tell you what I think, which is positive, then the easy accusation would be, 'Biden, as vice president, probably put pressure on one of the governing agencies to keep that from happening,'" Biden said, as political speculation swirled around the impromptu meeting between Attorney General Loretta Lynch and former President Bill Clinton on a Phoenix airport tarmac last Monday. "The president and I have kept our hands off it completely."

Biden also remarked that "she is incredibly by leaps and bounds more qualified than the other person to be the president of the United States."

"I’ve known her for 40 years; I’m a friend of hers," Biden said. "And I find it hard to believe that she would do anything intentionally wrong."

Biden’s comments aired a day after the Federal Bureau of Investigation hosted Clinton at its Washington, D.C., headquarters for what her campaign said was a “voluntary interview.”

After the interview, which a campaign aide said lasted for 3½ hours, Clinton told MSNBC's "Meet the Press" host Chuck Todd that she had been "eager to do it" and reiterated that she “never received nor sent any material that was marked classified” using her private email account.

Her prospective Republican opponent, Donald Trump, fired off a pair of tweets as news broke of the interview.

“It is impossible for the FBI not to recommend criminal charges against Hillary Clinton,” he wrote. “What she did was wrong! What Bill did was stupid!"

He added later: “It was just announced-by sources-that no charges will be brought against Crooked Hillary Clinton. Like I said, the system is totally rigged!”

Lynch said on Friday that she "certainly wouldn't" repeat the meeting with the former president, who nominated her to serve as U.S. attorney for the Eastern District of New York in 1999, remarking that she "fully" expects to follow the recommendations of career prosecutors in the case.

An aide to the former president told reporters on Saturday that while the meeting was “entirely social in nature,” he agreed with Lynch “that he would not do it again.”