Wisconsin Terrorist: Michael Page

(CNN) — Hundreds of white supremacist groups are active in the United States today and are experiencing a kind of resurgence, experts say.

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 gunman, who was killed, had ties to such a group.

Who are Sikhs?

“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 them and says it was tracking the Wisconsin gunman for years before the attack.

Wade Michael Page, 40, was a “frustrated neo-Nazi,” said the center. His tattoos included the Celtic cross adopted by white supremacist groups, and he was a member of two “racist skinhead” bands, it said.  Read further in CNN