“There are a few big groups and a lot of little ones, and they are growing in an echo chamber where all they hear is ‘anger, anger, anger, anger, anger,’ ” said J.J. MacNab, an author and George Washington University researcher who specializes in extremism.

Some of these groups espouse extremist, anti-government views, and their numbers jumped from 113 groups in 2014 to 180 last year, according to the Southern Poverty Law Center, which tracks extremism.

Ryan Lenz, an SPLC analyst, said that increase has partly been a response to a rise in white supremacist and white nationalist activity amid the racially charged environment of past two years, including the 2016 presidential campaign. For example, SPLC figures show that the number of Ku Klux Klan chapters increased from 72 in 2014 to 190 last year.