(CNN) - Quick. Think of a white supremacist group.
What comes to mind? The Ku Klux Klan during the 1920s? Maybe the Nazis in World World II?
Hundreds of white supremacist groups are active in the United States today and are experiencing a kind of resurgence.
They are in the spotlight this week in the wake of a deadly shooting at a Sikh temple outside Milwaukee. Authorities have said they are looking into whether the alleged gunman, who was killed, had ties to a white supremacist group.
"I know they're out there. I try not to alarm the public too much, but it's something that really needs to be taken a hold of now because more and more we see facts coming out that this is happening more often and more often, and it's not going to stop anytime soon," said David Gletty, who worked as an undercover informant on extremist groups for the FBI.
Currently, there are more than 1,000 hate groups in the United States, according to the Southern Poverty Law Center, which tracks such groups and says it was tracking the Wisconsin shooting suspect for years before the attack.