According to his former press secretary and trusted aide of seven years, Scott McClellan, President Bush knew very well that undercover CIA agent Valerie Plame was outed by White House leaks and may even have been party to the plan.

It seems, however, that everyone must wait until April, when McClellan’s book “What Happened” is published, to find out precisely what the president and Vice President Dick Cheney actually knew about the unmasking of Plame, which was committed allegedly in revenge for her ambassador husband Joseph Wilson’s revelation that part of the administration’s WMD claims were hokum.

All McClellan would say Tuesday at a “teaser” press conference for the book subtitled “Inside the Bush White House and What’s Wrong with Washington”, was that he had “unwittingly” passed on false information to the press that top aides Lewis “Scooter” Libby and Karl Rove were not involved in the Plame leaks. McClellan says that five people, including the president, vice president, Libby and Rove, knew this was false.

While the former press secretary’s desire to profit from a best seller is understandable, it is simply not good enough that he should be sitting on evidence of such fundamental importance. Indeed, it may be wondered where McClellan was when Libby was on trial for the leak. Libby was found guilty only on four lesser charges of blocking the subsequent investigation, fined $250,000 and sentenced to 30 months in jail. Bush immediately commuted this punishment because it was “excessive”.

If McClellan had told investigators then what he alleges now about the uncharged Rove, as well as Libby, then the outcome for both men might have been very different.

As puzzling as McClellan’s silence is this week’s press conference, fully five months before his book is due to hit the streets. Could it be that he felt obliged to go public to escape pressure being applied by the White House? McClellan may very well be a troubled man. He was Bush’s communications director when the president was governor of Texas. He ran three gubernatorial campaigns and was the key press aide during the 2000 presidential campaign. He served as principal White House press secretary from 2003 until last year. It may be that he saw and deplored the changes the presidency brought on his boss. Maybe he stayed on out of a blend of loyalty and pleasure at being so close to the seat of power. But, with his book, he has betrayed his old boss. Perhaps the more important question to ask is: Why?

McClellan is unlikely to be able to keep his revelations secret until the spring. If he is telling the truth then he is holding the Iraq invasion’s long-sought smoking gun. The Democrat-dominated Congress may subpoena him to give evidence to an enquiry. The federal prosecutor might want to hear his evidence about Rove and know why he kept quiet during Libby’s trial. No one will wait until April.