Forensic investigators have now been called in to investigate exactly what Nixon knew about the Watergate break- in, particularly the extent of his knowledge of the raids on the Democratic National Committee's offices in Washington.

Investigators appointed by the US National Archives are to analyse notes taken by the White House chief of staff HR Haldeman at a meeting with the late President just three days after Nixon campaign members were arrested for breaking into the Watergate building.

Their mission is to find out what Nixon and Haldeman discussed during the 18-and-a-half minutes missing from tape recordings of the meeting and from the aide's large yellow note book, the online edition of British newspaper 'The Daily Telegraph' reported.

Experts have given up trying to unlock the mystery from the erased tape. The search will instead scour Haldeman's notes for incriminating clues.

Investigators will use electrostatic detection analysis, which is capable of detecting and highlighting indented images, such as those left on a sheet of paper when a pen has written on a sheet above it. This might show evidence that certain pages were destroyed and even point to words so far lost to history, the report said.

Techniques known as hyperspectral imaging and video spectral comparison also will be used. And, the test results are expected early next year.

The prospect of confirming that a gap exists in the notes, corresponding with the gap in the recording, has Nixon historians on tenterhooks.

"My best scholarly guess is that Nixon asked Haldeman if anyone in the White House had advance knowledge of the Watergate break-in," Luke Nichter of Texas A&M University was quoted as saying