Sometimes a piece of propaganda is started almost by accident. Today's technology can help ill-founded rumours turn into fully-fledged lies that spread quickly around the world via the internet.

There is, for instance, one terrible untruth that is widely believed in the Arab and wider Muslim worlds, a lie that casts strange light on September 11th and that has influenced many people's thinking about that event.

The lie goes something like this: some 4,000 Jews didn't go to work in the World Trade Centre on September 11th, having phoned in sick. Therefore no Jews died in the atrocity. The only way this could have happened is if word had been spread among Jews to stay away from the buildings on that day. The most likely source of such an instruction would be the Israeli government. So Israel either knew about the attack, or even planned it as a way of discrediting Arabs.

This conspiracy theory, virtually unheard-of among ordinary people in the West, is a common explanation elsewhere. It is often used by Pakistanis, for example, who are anxious to clear Osama bin Laden of blame for the September 11th crimes.

Where did it come from? The online magazine Slate has investigated its origins and its spread. It appears that the 4,000 figure came from an Israeli embassy statement on September 11th expressing concern about the 4,000 citizens of Israel who live in New York (not who worked in the World Trade Centre). And it seems that a TV station in Lebanon, Al Manar, decided several days after the attack to broadcast a report that raised questions about what had happened to the 4,000 - either accidentally or deliberately confusing the question of who they were and where they lived or worked. Why, the report asked, did Israel first appear to mourn 4,000 dead, and then suddenly realise that they weren't dead at all? (Al Manar did not respond to Media Scope's request for information about its story.)

From there, the rumour spread quickly, with the help of e-mail and an Arab-oriented internet site called Information Times. An English-language story based on the report was flying around the world by September 20th, with many people using e-mail to forward it to dozens and hundreds of people at a time.

On September 21st, the story took a step into the media big time. A report appeared on the website of the hugely popular and influential Russian newspaper, Pravda. It was the same untrue story that had been doing the e-mail rounds, and it carried the byline of a Russian journalist who is based here in Ireland, Irina Malenko.

Malenko told Media Scope that she hadn't checked out the story herself. "I just translated it from English to Russian and passed it on to Pravda." She was annoyed, she said, when she found out that it had appeared with her byline, because she said she told her Pravda contacts that she had simply translated an e-mail.

Malenko's story was soon taken off the Pravda site, with the newspaper saying it had made "a great and terrible mistake".

However, the damage was done. The story had appeared in a reputable publication, aiding its further spread.