WASHINGTON — Former President Richard Nixon told a grand jury investigating the Watergate scandal that the 18 1/2-minute gap on one of the White House tapes was an “accident,” according to transcripts released Thursday.

Nixon, who had resigned his office under threat of impeachment 10 months before testifying in June 1975, offered no explanation for how the accident occurred.

The gap occurred on an audiotape of a conversation between Nixon and White House aide H.R. Haldeman that was subpoenaed as part of an investigation into the June 17, 1972, break-in at Democratic Party headquarters at the Watergate Hotel in Washington.

The transcripts were made public by the National Archives in Washington. In them, Nixon also defends his administration’s reaction to the leaking of the Pentagon Papers, which contained classified information about the Vietnam War, and mentions political dirty tricks he believed had been used against him.

The tape gap spurred suspicions that incriminating conversations about the break-in and subsequent cover-up among administration members had been intentionally erased. Nixon’s secretary, Rose Mary Woods, said at the time that as she had been transcribing the conversation she accidentally erased a few minutes by hitting the wrong button on the tape recorder when she answered the phone.

Woods “always denied that the buzz that she heard” after her mistake “was no more than four-and-a-half to five minutes, and she cannot explain how 18 minutes could have occurred,” Nixon testified under oath. “If you are interested in my view as to what happened, it is very simple. It is that it was an accident.”