As of this writing, investigators have not yet determined a motive for the crime. However, witnesses have indicated that the shooter had a 9/11 tattoo on his arm.
So, of course, there is speculation that this may have been a "revenge" shooting, even though Sikhs are not Muslims - they are two distinctly separate religions. (Of course, even if they were Muslims, this crime would be no more justified.)
Distinctions, however, are often lost on bigots who just see the world in terms of "us" vs. "the other". Sikhs wear turbans, and some al-Qaeda members have been seen on TV wearing turbans, so that's close enough. Brown skin is brown skin, and that is enough to earn the label of "other". An exotic style of dress further fuels the fire of hatred.
This kind of ignorance and blind prejudice disgust and embarrass me. My heart goes out to the Sikh community in the wake of yesterday's tragedy. I apologize for the ignorance and intolerance that some in this country force you to endure.
Update (08 Aug): It now appears that the shooter, Wade Michael Page, was not killed by police but rather died of a self-inflicted gunshot to the head after being shot in the stomach. What a coward!