Chips smaller than fingernails helped nab dozens of terrorists for two years, a report said on Thursday.

Investigators, it said, were able to track conversations and movements of several Al Qaeda leaders and operatives after determining that the suspects favoured a particular brand of cell phone chips that carry prepaid minutes.

The investigators said they believed that the chips, made by Swisscom of Switzerland, were popular with the terrorists because they could buy them without giving their names.

"They thought these phones protected their anonymity, but they didn't," an official based in Europe was quoted as saying. Even without personal information, the authorities were able to monitor conversations.

The New York Times report said the investigation, codenamed Mont Blanc, began almost by accident in April 2002, when authorities intercepted a call that lasted less than a minute and involved not a word.

Investigators, suspicious that the call was a signal between terrorists, followed the trail first to one terror suspect, then to others, and eventually to terror cells on three continents.

What tied them together was the chip. And before the investigation wound down in recent weeks, its global net caught dozens of suspected Al Qaeda members and disrupted at least three planned attacks in Saudi Arabia and Indonesia, officials in Europe and the US told the paper.

The investigation helped narrow the search for one of the most wanted men in the world, Khalid Sheikh Mohammed, who is accused of being the mastermind of the September 11 attacks, according to three officials based in Europe.

American authorities arrested Mohammed in Pakistan last March.

A half dozen officials in the US and Europe, the paper said, agreed to talk in detail about the previously undisclosed investigation because, they said, it was completed.

They also said they had strong indications that terror suspects, alert to the phones' vulnerability, had largely abandoned them for important communications and instead were using email, Internet phone calls and hand-delivered messages.

"This was one of the most effective tools we had to locate Al Qaeda," said an official in Europe. "The perception of anonymity may have lulled them into a false sense of security. We now believe that Al Qaeda has figured out that we were monitoring them through these phones."

The officials called it one of the most successful investigations since September 11, 2001, and an example of cooperation between agencies in different countries.

Led by the Swiss, the investigation involved agents from more than a dozen countries, including the US, Pakistan, Saudi Arabia, Germany, Britain and Italy.