After the huge response to Richard Porter's blogs last year about 9/11 Part of the conspiracy? I was very keen to get to the bottom of what exactly happened.

For the latest Conspiracy Files programme, on 9/11 - The Third Tower (Sunday 6 July at 2100 BST on BBC Two), I've been looking in detail at allegations that there was a conspiracy to deliberately demolish a third tower at the World Trade Center.

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This third skyscraper was never hit by an aeroplane. There is little photographic evidence of extensive damage. Yet seven hours after the Twin Towers collapsed, this 47-storey building collapsed in a few seconds.

Afterwards the thousands of tonnes of steel from the building were taken away to be melted down in the Far East. The official explanation is that this third huge tower at the World Trade Center collapsed because of ordinary fires - but that makes this the first and only skyscraper in the world to have collapsed because of fire. Nearly seven years on the final official report on the building has still not been published. The report is now promised this month.

World Trade Center Building 7 has become the subject of heated speculation and a host of conspiracy theories suggesting it was brought down by a controlled demolition. And some people suggest it was not just the government and foreign intelligence, but the police, the fire service, first responders and even the media that were involved.

It is certainly true that on 9/11 the BBC broadcast that WTC7 had collapsed when it was still standing. Then the satellite transmission seemed to cut out mysteriously when the correspondent was still talking. Then Richard Porter admitted in his blog last year that the BBC had lost those key tapes of BBC World News output from the day.

So is that proof that we at the BBC are part of a huge sinister conspiracy or is there a simpler explanation?

The mystery of the missing tapes didn't last that long. One very experienced film librarian kindly agreed to have another look for us one night. There are more than a quarter of a million tapes just in the Fast Store basement at Television Centre. The next morning I got a call to say the tapes had been found. They'd just been put back on the wrong shelf - 2002 rather than 2001. Not so sinister after all.

What about the incorrect reporting of the collapse of Tower 7? Having talked to key eyewitnesses who were actually at Ground Zero that day it is clear that, as early as midday, the fire service feared that Tower 7 might collapse. This information then reached reporters on the scene and was eventually picked up by the international media.

The internet movie Loose Change has been viewed by more than 100 million people according to its makers and it asks this question in the latest film release: "Where did CNN and the BBC get their information especially considering the building was still standing directly behind their reporters?"

It turns out that the respected news agency Reuters picked up an incorrect report and passed it on. They have issued this statement:

"On 11 September 2001 Reuters incorrectly reported that one of the buildings at the New York World Trade Center, 7WTC, had collapsed before it actually did. The report was picked up from a local news story and was withdrawn as soon as it emerged that the building had not fallen."

I put this to the writer and director of Loose Change, Dylan Avery. I asked whether he believed the BBC was part of the conspiracy. Given the question his film had posed about the BBC I was surprised by Dylan's response: "Of course not, that's ludicrous. Why would the BBC be part of it?"

He added candidly: "I didn't really want to put that line in the movie."

And the reason the interview with the BBC correspondent, Jane Standley, ended so abruptly? The satellite feed had an electronic timer, which cut out at 1715 exactly.

We've done our best to tackle many of the other questions raised about Tower 7. I interviewed the lead official investigators, scientists and eyewitnesses who support the official explanation; but also architects, engineers and others who now question that account.

The final report on 9/11 should be with us soon. The official investigators are confident they will be able to solve the final mystery of 9/11. But I doubt they will ever convince their harshest critics, who believe there was a home-grown conspiracy at work that day.

