WASHINGTON (Reuters) - Homeland Security Secretary Janet Napolitano warned Wednesday that the threat of terrorism against the United States was in some ways at its most heightened state since the September 11, 2001 attacks. In addition to the threats by Al-Qaeda, Napolitano said the country faces new threats from those inspired by the group and those already inside the United States. The threat continues to evolve and in some ways the threat today may be at its most heightened state since the attacks nearly 10 years ago, Napolitano told the US House of Representatives Homeland Security Committee. She also said in her testimony to lawmakers that US officials believed there may be individuals who want to carry out attacks already in the country and that they could carry out acts of violence with little or no warning. Individuals associated with Al-Qaeda and the Taliban have tried to carry out several attacks against the United States, including by a Nigerian man who allegedly tried to blow up a US airliner with a bomb hidden in his underwear and another individual who plotted to attack the New York subway system. As I have said before, we cannot guarantee that there will never be another terrorist attack, and we cannot seal our country under a glass dome, Napolitano said. However, we continue to do everything we can to reduce the risk of terrorism in our nation. The head of the National Counterterrorism Centre, Michael Leiter, told the committee that the Al-Qaeda off-shoot based in Yemen, Al-Qaeda in the Arabian Peninsula (AQAP), at the moment represented the biggest threat to the United States. Leiter said that the parent Al-Qaeda group, believed to be hiding in the Pakistan-Afghanistan border region, was probably at its weakest point since the September 11, 2001 attacks but remained a very determined enemy. I actually consider Al-Qaeda in the Arabian Peninsula with (Anwar) al-Awlaki as a leader within that organization probably the most significant risk to the US homeland, Leiter told the committee, noting that it has a large Internet following. Al-Awlaki, a Muslim cleric who is 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 last two years. The group has claimed responsibility for the 2009 Christmas Day thwarted attack aboard a US airliner and a more recent attempt to blow up two US-bound cargo planes with toner cartridges packed with explosives. Al-Awlaki also communicated with a US Army Major Nidal Malik Hasan who in November 2009 allegedly went on a shooting rampage at Fort Hood, Texas, that killed 13 and wounded 32. Leiter said that it appeared to be more inspiration rather than direction.