NEW YORK (Reuters) - Jimmy Walter has spent more than $3 million (1.5 million pounds) promoting a conspiracy theory the September 11, 2001, attacks on the United States were "an inside job" and he is offering more cash to anyone who proves him wrong.

The millionaire activist is so convinced of a government cover-up he is offering a $100,000 reward to any engineering student who can prove the World Trade Centre buildings crashed the way the government says.

"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 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 $1,000. Winners will be chosen next June.

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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 wasn't 19 screw-ups from Saudi Arabia who couldn't 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.'"...