Bin Laden's deputy 'plotted gas attack' on New York subway

An al-Qa'ida cell in the United States came within 45 days of launching a cyanide attack on the New York subway system that could have killed as many people as the attacks of 11 September 2001, according to a new book given broad credence by American police and intelligence officials.

The attack was called off by Osama bin Laden's deputy, Ayman al-Zawahiri, for reasons that remain unclear, and the man masterminding the operation from Saudi Arabia - codename Swift Sword - was later killed in a shootout with Saudi security forces. The book, The One Percent Doctrine: Deep Inside America's Pursuit of Its Enemies Since 9/11, carries unusual authority since it was written by Ron Suskind, one of America's most respected and best-informed investigative reporters. Chillingly, it suggests that al-Qa'ida has cracked the long-standing problem of how to deliver poison gases like cyanide for maximum destructive effect, and offers no reassurance that the cell that was ready to attack New York in 2003 has been disbanded or left the country.

In late 2002, a joint US-Saudi counter-terrorism operation uncovered the existence of a device nicknamed the mubtakkar - Arabic for "invention". The mubtakkar contains two chambers keeping the ingredients for hydrogen cyanide both stable and separate until the attackers are ready to break the seal between the chambers and initiate the attack - possibly by remote control.

The mubtakkar is small enough to be transportable in a backpack or small bag. The detonation of several of them in different cars of the same subway train at rush hour could conceivably cause hundreds or even thousands of casualties, according to Suskind's sources.

The book describes the rank fear inside the White House following the decision to pull the plug on the attack. "What does calling this off say about what else they're planning?" he quotes President George Bush as saying. "What could be the bigger operation Zawahiri didn't want to mess up?" Not everyone, however, agrees that such fear was necessarily justified. Yesterday's New York Times quoted intelligence experts casting doubt that the mubtakkar really worked as advertised. "They'd be lucky if they killed everybody on one car - you can do that with a 9-millimetre pistol," one official told the paper.

Suskind's book also paints a fascinating picture of the tense, unreliable nature of US-Saudi co-operation on counter-terrorism.

No episode illustrates this better than the hunt for Swift Sword, the group's chief operator on the Arabian peninsula whose real name is believed to be Yusef al-Ayeri. The Saudis had Ayeri in their custody in late 2002 but released him apparently without appreciating his significance.

US officials realised that Ayeri was the only person who could unravel details of the New York cyanide plot and made his capture a priority. In May 2003, though, Ayeri was one of a carful of young men who ran a roadblock near Mecca and tossed a grenade at the uniformed guards. A shoot-out ensued, in which everyone, including Ayeri, was killed. Ayeri was carrying a personal letter from bin Laden.

The Saudis took several days to tell the US about the shoot-out and failed to recover themobile phone and address book he had on him. They also failed to follow up the shoot-out with raids on properties linked to the dead men.