WASHINGTON (AP) – What was recorded during the 18½-minute gap of one of President Nixon’s tapes will remain a mystery – at least for now.

The National Archives says audio experts were unable to recapture intelligible sounds from test tapes that simulated the recording made famous in the Watergate scandal.

“I am fully satisfied that we have explored all of the avenues to attempt to recover the sound on this tape,” U.S. archivist John Carlin said Thursday. “We will continue to preserve the tape in the hopes that later generations can try again to recover this vital piece of our history.”

In 2001, the archives created a panel of experts to determine whether advances in the field of forensic audio technology could recover what was on the tape recorded three days after the break-in at the Democratic National Committee headquarters in the Watergate. Based on disappointing results of two tests, Carlin decided not to continue the effort to turn the gap – a series of clicks, hisses and buzzes – into intelligible speech.

The gap is part of a recording made June 20, 1972, in the old Executive Office Building as Nixon chatted with his chief of staff, H.R. Haldeman.

News of the erasure, late the following year, eroded Nixon’s credibility at a time when his presidency was unraveling over the June 17, 1972, break-in.

Nixon’s secretary, Rose Mary Woods, testified that she was transcribing the tape when a phone rang.

She said she must have pushed the wrong button and left her foot on a pedal, accidentally recording over part of the original conversation.

A panel of experts set up in the 1970s by federal judge John Sirica, who presided over the Watergate criminal trials, concluded that the erasures were done in at least five – and perhaps as many as nine – separate and contiguous segments. The panel never figured out what was erased.

Experts were not allowed to do tests on the original tape.

, which is stored at 65 degrees in a vault at the archives in College Park, Md.

The had to prove they could retrieve voices from test recordings that have been erased without damaging the test tapes, said Susan Cooper, a spokeswoman for the archives.

The first test tape, distributed to experts in February 2002, consisted of erased test tones and spoken words. Results included partial identification of test tones, but none of the spoken words were recovered.

A second test tape was sent to experts in August 2002. This tape was recorded using blank 0.5-millimeter tape confiscated from the Nixon White House. It contained speech similar in quality to speech heard before and after the gap on the tape. None of the experts recovered any intelligible audio from the second test tape, the archives said.

Paul Ginsberg, president of Professional Audio Laboratories Inc., in Spring Valley, N.Y., was one of five experts who worked on the project. He used a machine that reviewed about 60 different sections across the width of the tape, especially the edges that might not have been fully erased.

“Quite frankly, I was only able to recover a hum, or actually a buzz, consistent with a malfunctioning machine,” he said.

“Was I disappointed? Yeah. I wanted to hear the conversation. I’ve got a pretty good track record and I’m used to getting the puzzle solved.”

Steve St. Croix, an audio expert and president of Intelligent Devices in Owings Mills, Md., said he was able to retrieve digital data from the test tapes, but that post-Sept. 11 projects involving national security made it impossible to spend the time needed to analyze the data.

“To do it really, really properly is a long-term project that we just couldn’t get involved with due to the changing complexities of the world,” St. Croix said, adding that he’d like to continue trying in the future.

James B. Reames at JBR Technology Inc., a video forensic laboratory in Springfield, Va., said his company recovered a tone off the first test tape and several “unusual sounds,” but no understandable words, from the second test tape. He has asked the archives if he can have the test tapes back so he can keep working on them in hopes of one day making a breakthrough.

The gap on the tape “must have meant something to somebody or they wouldn’t have erased it,” Reames said.

AP-ES-05-08-03 1628EDT