To the disappointment of history buffs, the most famous 18 1 / 2 minutes in U.S. history remain a mystery.

High-tech detective work undertaken two years ago by the National Archives the Library of Congress and other government agencies to unravel what was said during 18 1 / 2 minutes that were erased from the tapes in the Watergate scandal has come up dry, Archives officials said Thursday.

“It was a fun process,” said David Paynter, an archivist with the Special Access and Freedom of Information Act staff. “It would have been interesting to come up with something.”

Three days after the break-in at the Democratic National Committee headquarters on June 17, 1972, President Richard M. Nixon and his chief of staff, H.R. Haldeman, met at the White House to discuss the incident.

Eighteen-and-a-half minutes were mysteriously erased from a tape recording of that meeting. The question of what Nixon knew and why caused a national controversy when investigators discovered the blank recording.

The only meeting record is two pages of notes taken by Haldeman, and for years, historians believed the papers might hold the clue to what was erased from the famous audiotapes that led to Nixon’s resignation.

After several efforts to salvage the lost audio failed, historians turned their focus to Haldeman’s handwritten notes. Two years ago, Archives officials were contacted by Phil Mellinger, an amateur historian from Ellicott City, Md., whose intelligence background led him to propose a high-tech forensic analysis. Mellinger believed that Haldeman may have destroyed notes on additional pages.

A ballpoint pen may have left faint impressions on the existing pages that could be deciphered using a technique called electrostatic detection analysis.

The Archives got to work with a team of forensic document examiners from the agency, the Library of Congress, the Treasury Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The team met for three days in late 2009, placing the original notes under a thin plastic film, applying a charge and brushing a substance similar to black copier toner over the documents. If there are indented impressions, black particles are supposed to reveal them.

The test turned up nothing — except that Haldeman used different pens to write the date and page numbers in the body of the text.

“We’ll never know what that means,” Paynter said. A handwriting analysis also was performed, confirming that Haldeman wrote the notes.

It took nearly 18 months for the agencies involved to write up their notes, a delay that Paynter attributed to the pressure on volunteers.

In 2001, the Archives assembled an expert panel to determine whether advances in the field of forensic audio technology could recover what was on the tape. But they were unable to recapture the words.

Nixon’s secretary, the late Rose Mary Woods, ended up taking the blame for the erasures.