And he is offering more cash to anyone who proves him wrong.
The millionaire activist is so convinced of a government cover-up that he is offering a $100,000 reward to any engineering student who can prove the World Trade Centre buildings crumbled the way the government says they did.
"Of course, we expect no winners," Walter, 57, heir to an $11 million fortune from his father's home-building business, said in a telephone interview from California on Wednesday. He accuses figures in government, the military and business of involvement in the September 11 attacks.
Panel of judges
Walter said a panel of expert engineers would judge submissions from the students.
Next month, he also launches a nationwide contest seeking alternative theories from college and high school students about why New York's World Trade Centre collapsed. The contest offers $10,000 to the best alternative theory, with 100 runner-up awards of $1000. Winners will be chosen next June.
The attacks in New York and
Washington killed 2749 people
The World Trade Centre's twin towers were destroyed after hijackers from Usama bin Ladin's al-Qaida armed group slammed two commercial airliners into them.
Various official investigations give no credence to Walter's theory. A September 11 commission spokesman said its policy was not to comment on criticism of the report.
Walter insists there had to be explosives planted in the twin towers to cause them to fall as they did, and also rejects the official explanation for the damage done at the Pentagon.
"We have all the proof," said Walter, citing videotapes and testimony from witnesses. "It was not 19 screw-ups from Saudi Arabia who could not pass flight school who defeated the United States with a set of box-cutters," he said. He dismissed the official September 11 commission report, saying: "I don't trust any of these facts."
Walter has spent millions of dollars to bolster support for his case, running full-page adverts in The New York Times, the Wall Street Journal, The New Yorker and Newsweek, as well as alternative newspapers and 30-second TV spots.
He points to a Zogby poll he commissioned last summer that showed 66% of New Yorkers wanted the 9/11 investigation reopened.
"I am a patriot fighting the real traitors who are
destroying our democracy. I resent it when they call me delusional"
Walter has spent about 30% of his net worth on his efforts.
"I am a patriot fighting the real traitors who are destroying our democracy. I resent it when they call me delusional," he said.