U.S. counter-terrorism authorities have identified a leading Saudi-born al-Qaida operative as a key figure in a September 2009 plot to bomb New York's subway system, an attack that could have matched the carnage of 9/11.

Adnan el Shukrijumah, 34, is a naturalized U.S. citizen but he has a $5 million bounty on his head, a measure of the threat U.S. authorities consider he poses. He is linked directly to al-Qaida's top leadership and the subway plot underlines how they are still striving to pull off another cataclysmic assault on the United States.

Dozens of plots to attack targets in the United States have been thwarted or unraveled since 2001. Most have been do-it-yourself conspiracies by jihadist cells or individuals acting independently of al-Qaida's core leadership holed up in Pakistan.

But some, like the New York subway plot that involved three suicide bombers, clearly have engaged Osama bin Laden and his lieutenants who have long sought to unleash a 9/11-level attack in the United States.

In 2004, U.S. Attorney General John Ashcroft described Shukrijumah as a "clear and present danger" to the United States.

Abu Zubaydah, identified by U.S. officials as a top al-Qaida commander who was captured in Faisalbad, Pakistan, on March 28, 2002, told his interrogators that Shukrijumah was one of the most likely al-Qaida operatives to attack the United States or Western Europe.

The last successful al-Qaida attack in the West was the July 7, 2005, suicide bombings against London's transport system. Three subway trains and a double-decker bus were hit, killing 52 people.

Shukrijumah, who is believed to be based in Pakistan, was charged on July 7 with directing the failed New York subway plot and involvement in a similar conspiracy to bomb London and Manchester that was foiled by British security authorities in 2009.

He has been high on the U.S. wanted list after he was named by Khaled Sheik Mohammed, al-Qaida's operations chief captured in Rawalpindi, Pakistan, on March 1, 2003.

But the 5-foot-4 jihadist has eluded a global manhunt for eight years. He's a master of disguise who speaks half a dozen languages, including American English with a trace of accent. He is said to carry multiple passports, including Saudi Arabian, Canadian, Guyanese and Trinidadian documents.

He's considered one of the five most dangerous terrorist fugitives, largely because having lived in the United States for years he is able to move around in Western society without arousing suspicion.

For several months in 2003, there was a flurry of reported sightings in Latin America and the Caribbean but U.S. authorities say they believe he went to ground in Pakistan's lawless tribal belt.

However, last summer the CIA said he was traveling around the Middle East using an alias and a Saudi diplomatic passport. Riyadh denied that.

The eldest of five children, Adnan Ghulshair el Shukrijumah was born Aug. 4, 1975, in Medina, Saudi Arabia. His father Gulshair, was a Guyanese and a radical Muslim cleric, his mother Zuhrah was Saudi Arabian.

When Shukrijumah was young, his father was assigned to Trinidad and Tobago as a Muslim missionary. The family moved to Florida in 1985 and in 1996 settled in Miramar, a Fort Lauderdale suburb. Shukrijumah studied computer engineering at Broward Community College.

There he became friends with Jose Padilla, who would achieve notoriety in May 2002 when he was arrested in Chicago for allegedly planning to detonate a radioactive uranium-enriched bomb in 2002. That charge was eventually dropped but Padilla was sentenced to 17 years in 2008 for aiding terrorists.

After 9/11, Shukrijumah came under suspicion because of his links to a Muslim named Imran Farouk Mandhai, who was later convicted of trying to blow up South Florida power plants.

But the FBI apparently had no idea of his real status until they were able to unravel the skein of aliases -- at least six -- that he used.

Abu Zubaydah told his interrogators after being waterboarded in late 2002 of several al-Qaida operations he knew of. One involved "Jaafar al Tayyar," Arabic for "Jaafar the pilot," who he said would deliver an "American Hiroshima" using radioactive bombs.

But it wasn't until Khaled Sheik Mohammed, who had met Shukrijumah in Pakistan, spilled the beans that they realized the fugitive had lived in Florida for 16 years.