Boston ? President Nixon had no sinister purpose when he made the fateful decision to have six microphones installed in his Oval Office desk, the former aide who revealed the existence of the recordings said Sunday at a conference on presidential tapes.

“It was simply for history,” Alexander Butterfield said. “He really cared about history.”

The taping of presidential conversations began with Franklin D. Roosevelt, but became public knowledge only when Butterfield, testifying before a congressional panel during the Watergate scandal, revealed that Nixon had taped conversations in the Oval Office and in his Cabinet Room. Nixon’s taping system, installed by the Secret Service in 1971, eventually helped bring down his presidency.

“We marveled at his ability to seemingly be oblivious to the tapes,” said Butterfield, who added that he and other staffers often would ask themselves, “‘He’s not really going to say this, is he?’ But he did.”

Butterfield said he believes Nixon fully implicates himself in the Watergate break-in during the famous 18-minute gap in his tapes. Efforts are under way to retrieve the sound of the erased tapes.

“At this point, the jury is still out as to whether that technology exists,” U.S. Archivist John Carlin, a former Kansas governor, said.

President Ford, who had the microphones removed after Nixon resigned in 1974, pardoned his predecessor in part because the legal status of the tapes was uncertain, former Ford aide James Cannon said.

“He brooded about it for a couple of days. His decision was, the only solution was, to sweep it off the table, and that led to the pardon,” Cannon said.

Presidents from FDR to Nixon made tapes to varying degrees, but no recent presidents have done so, Carlin said. Given Nixon’s experience, it could be political suicide, he added.