In her defence and in the interests of balance, she was normally right. Most mass shootings in the US are done by white men. For much the same reason most mass shootings in China would be done by Asians. They're the dominant demographic.

She took an intuitive punt without stopping to analyse the details. Once you correct for population percentages, race doesn't really correlate with shootings at all. Her comment was racially prejudiced and stupid for someone in such a position. Nothing to do with white supremacy and everything to do with gross misconduct.

You can read more here if you're interested.

https://medium.com/lucid-nightmare/mass-shootings-arent-about-racism-1eec3db4a708