On two occasions in the last few days the United States government urged its nationals abroad to take special security precautions. The first warning came on Monday, after a Pakistani was convicted in the US for the murder of two CIA employees. The subsequent killing of four Americans in Pakistan suggested that the threat of revenge attacks was real. On Wednesday Washington issued another warning to expatriates when a court in New York found two Arabs guilty of planning the 1993 bombing of the World Trade Centre. The main defendant is this case is Ramzi Ahmed Yousef, whom the US authorities have described as representing a new breed of terrorist. The BBC's US Affairs Analyst, Henri Astier, looks at his background.
According to the US authorities, Ramzi Ahmed Yousef embodies a new threat faced by American interests abroad. The anti-western guerrillas of the past belonged to familiar -- if dreaded -- groups backed by states and fighting for well-defined political goals. Yousef, by contrast, is deeply religious; he does not appear to rely on a sophisticated network of sponsors; and according to the prosecutors, his main aim has been to kill as many Americans as possible.
Yousef is indeed a mysterious figure. It's thought he was born twenty-nine years ago in Pakistan. He has travelled at various times under forged passports from Iraq, Pakistan, Italy and Britain; he himself says that his mother was a Palestinian, his father a Pakistani, and that he grew up in Kuwait. But this has not been confirmed and even his real identity is unclear.
US investigators say Yousef studied engineering in Wales in the 1980s, and became an expert in electronics and explosives. After that it's believed he trained Muslim guerrillas in the Philippines and Pakistan. His main support during those years appears to have been a wealthy Saudi widely believed to be supporting fundamentalist groups in various countries
Prosecutors say Yousef flew to New York in 1991, carrying an Iraqi passport. He gained the confidence of four local Muslims who were followers of the blind Egyptian cleric, Sheik Omar Abdel-Rahman. The four men were later convicted of murder for the death of six people in the World Trade Center bombing in 1993.
But the US authorities believed Yousef was the mastermind of the operation. They say he took a Jordanian airliner to Amman immediately after the bombing. In 1994 he surfaced again in Pakistan and the Philippines. According to the Philippine and American governments, he planted a bomb which killed one passenger on Philippine Airlines flight.
After the attack Yousouf again eluded police. But two of his associates were arrested and the Philippine police found a cache of documents suggesting that Yousef was planning to bomb twelve US airliners, killing thousands of people in a single "day of hate".
By that time Yousef was one of the FBI's most-wanted fugitives. His luck ran out after his return to Pakistan in 1995. When an acquaintance turned him in, he was captured by US agents and flown back to the US. At that time he sent a rambling statement to the New York Times, saying that he was a holy warrior and that Muslims had a right to punish the US for its support of Israel.
Last year Yousef was convicted in connection with the abortive plot to bomb US airliners. Now he's been found guilty of organising the World Trade Centre Bombing. And the US government is hoping his conviction will not spark further attacks against US interests by other freelance guerrillas.