According to a Government Accountability Office (GAO) report, it’s the latter and by a very wide margin. From p. 4 of GAO’s report:
Of the 85 violent extremist incidents that resulted in death since September 12, 2001, far right wing violent extremist groups were responsible for 62 (73 percent) while radical Islamist violent extremists were responsible for 23 (27 percent).
But as researchers at the Georgia State recently reported, media coverage of terrorist incidents makes it seem as if terrorism is almost exclusively perpetrated by Muslims:
We examined news coverage from LexisNexis Academic and CNN.com for all terrorist attacks in the United States between 2011 and 2015. Controlling for target type, fatalities, and being arrested, attacks by Muslim perpetrators received, on average, 449% more coverage than other attacks. Given the disproportionate quantity of news coverage for these attacks, it is no wonder that people are afraid of the Muslim terrorist.
More representative media coverage could help to bring public perception of terrorism in line with reality.
That incident-media reporting disconnect is matched by another: the notion that Arab/Muslim-Americans are more susceptible to radicalization, and thus to becoming terrorists, and that there are a discreet set of reliable indicators that will tell authorities who is or is not more likely to become a terrorist.