Yemen-based Al Qaeda offshoot now represents the biggest threat to the US as terrorist cells in Pakistan and Afghanistan weaken; US experts have told a congressional hearing. US secretary of homeland security Janet Napolitano said the threat from the troubled border region of Pakistan and Afghanistan was probably at its weakest point since September 11, 2001. But she warned the threat of terrorism against the United States was in some ways at its most heightened state since the attacks on September 11, 2001. And the Yemeni chapter of Al Qaeda – led by radical US-born cleric Anwar al-Awlaki and alleged to include Australians among its ranks – now constitutes the most significant threat to the US. Al-Awlaki, a Muslim cleric who is a US citizen but left the country in 2001 and joined Al Qaeda in Yemen, has been tied to plots against the United States over the past two years. The group has claimed responsibility for the thwarted 2009 Christmas Day attack aboard a US airliner and a more recent attempt to blow up two US-bound cargo planes with toner cartridges packed with explosives.