Almost nine years after the Sep 11 2001 terror attacks, a worrying number of American citizens and longtime US residents are becoming radicalised and training to be terrorists in Pakistan, US counter-terrorism officials say. Brainwashed by Al Qaeda's extremist ideology, many are also learning the bomb-making skills necessary to become potentially dangerous terrorists, CNN said in a special report citing officials. They are training in the mountains of Waziristan in northwestern Pakistan, where Al Qaeda still enjoys significant safety, it said. That's where, according to the US government, alleged Times Square bomber Faisal Shahzad was trained by the Pakistani Taliban, a group with close ties to Al Qaeda. Shahzad's case has strong similarities to that of another American, Bryant Neal Vinas, who plotted with terrorist groups in Pakistan to attack the US, CNN said. Vinas, a Catholic convert to Islam from Long Island, New York, who became radicalized, travelled to Pakistan to join up with Al Qaeda and helped Osama bin Laden's terrorist organization plot a bomb attack on New York City. CNN said both Vinas and Shahzad were well-integrated into American life before becoming radicalised. Both travelled to the heart of Al Qaeda's operational command in Pakistan's tribal region along the border with Afghanistan. And both allegedly met with the most senior leaders of the Pakistani Taliban in the weeks before allegedly plotting against the United States. Several top US counter-terrorism officials cited by CNN had the same message: Americans radicalised at home and trained in Pakistan represent a new and disturbing threat to the American homeland. In the last year, 16 Americans or American residents were implicated in Islamist terrorism, a surge in such cases. The Times Square plot is case No. 17. While it is still unclear whether Shahzad had radical associates in the United States, counterterrorism officials are concerned that others like him may be being radicalized through personal contact with proselytisers, CNN said. Most serious plots directed at the West in the last six years saw plotters either trained or directed by established jihadist groups in Pakistan, according to a recent study conducted for the New American Foundation. In recent months, videos have emerged purporting to show two Americans fighting with militants along the Afghanistan-Pakistan border. One of the alleged Americans, going by the name Sayfullah Amriki, was featured with his face blurred in a video produced by the propaganda arm of the Islamic Jihad Union, an Al Qaeda-affiliated Uzbek group. In the video, Amriki said he was not the only American who had joined up with militants in the area. He also made a plea in English for new recruits to fight American forces in Afghanistan. "We must rush to the lands to jihad. It is an obligation on us," he said. "How can we lose when we wish for death?" But it's what happens when fighters like Amriki come home that most worries US counterterrorism officials, CNN said.