This goes back to before the US was even a country. Still, the number of terrorist activities by the Christian Americans are a lot higher than Muslim terror attacks.
There have been terrorist attacks within the US nearly every day that include several people killed or wounded by one or more terrorists. We don't think about it because the news and government don't want to identify these killers as being the home grown sort. Most use firearms to kill innocent people, although other methods are used in various incidents. Explosives have been popular for much of the last 100 years, especially where the terrorists of organized crime are involved. They make local news, but rarely go national, but it's terrorism none the less. The drug wars being fought in our cities are also terrorism, but mostly ignored and never labeled as such.
Who's doing all this terrorism. For the most part, it's males, regardless of religion. Tate a female is perpetrating them, and usually, they are following the lead of their male lover.
Religion? With the majority of Americans claiming some affiliation with one of the myeiad of Christian faiths, it's only natural that the majority of mass killings in the US can point to Christian terrorists. Not all, but a majority.
Race? Again, it's a toss up. The majority of the mass killings in the US today belong to the various crime organizations, many which are ethnic based, and are not done for political jihad, but for money and power. Others are due to mental illness, domestic disputes that have gone too far, and, the religious right wing extremists that believe they are doing God's work by blowing up abortion clinics or bombing churches attending by minorities. It's terrorism.