Sources tells CBS News that investigators are continuing to look for any possible ties between the alleged Fort Hood shooter, Major Nidal Malik  Hasan, and terrorist groups including the Muslim Brotherhood, an affiliate of Hamas.

The Muslim Brotherhood operates behind the scenes in the United States and seeks to subtly place individuals into influential positions in the academic, legal or medical professions to further their cause of undermining the United States.

Investigators are also looking at whether Hasan was supporting Islamic charities overseas--charities which may have been funneling money to terrorist organizations.

While officials continue to probe Hasan's possible ties to terrorists, at this point they still believe strongly that he acted alone.