New York: A tiny chip smaller than a fingernail helped counter terrorism and nab dozens of terrorists for two years, a media report said.

The investigators, it said, were able to track the conversations and movements of several al-Qaeda leaders and dozens of operatives after determining that the suspects favoured a particular brand of cellphone chip. The chips carry prepaid minutes and allow phone use around the world.

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

"They thought these phones protected their anonymity, but they didn't," a senior intelligence official based in Europe was quoted as saying. Even without personal information, the authorities were able to conduct routine monitoring of phone conversations.

The 'New York Times' report said investigation, code-named Mont Blanc, began almost by accident in April 2002, when authorities intercepted a cellphone call that lasted less than a minute and involved not a single word of conversation.

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 computer 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, counter terrorism and intelligence officials in Europe and the US paper said.

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

American authorities arrested Mohammed in Pakistan last March.

A half dozen senior officials in the US and Europe, 'The Times' 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 e-mail, internet phone calls and hand-delivered messages.

"This was one of the most effective tools we had to locate al-Qaeda," said a senior counter-terrorism 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 the operation one of the most successful investigations since September 11, 2001 and an example of unusual 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.