Are the lectures of Shaykh Anwar al Awlakki authentic?

With regards to Shaykh Anwar al Awlakki, you need to be a bit cautious as he used to hold some extreme opinions which are not considered valid by the mainstream Muslims. According to him, it was permissible to attack America even if American civilians are killed in the process since they elected George Bush for the second term as a President. I am aware that he has a very good series on Seerah and life of the companions, but still you need to take care of the authenticity of the narrations used by him as sometimes he even uses weak narrations in the flow.