KANSAS CITY, Mo. — The seizure of a federal wildlife refuge in Oregon by armed anti-government extremists has attracted national attention and raised questions about whether the takeover is a powder keg about to blow.

But experts who monitor the anti-government movement say an even greater concern is lurking behind the confrontation that erupted Saturday at the Malheur National Wildlife Refuge in the high desert of eastern Oregon.

The militia movement, they say, is in the midst of a massive growth spurt at levels even greater than in the early 1990s after the Waco, Texas, standoff and the siege at Ruby Ridge, Idaho. It peaked after the Oklahoma City bombing.

“It’s bigger than anything we’ve seen before,” said Leonard Zeskind, president of the Institute for Research and Education on Human Rights.

“And it’s not simply a resurgence of the ’90s militia movement; it’s different in many ways. Now it’s much broader.”

The growth of an “insurgent militia movement,” Zeskind said, is the result of a combination of events, including a renewed effort to strengthen gun control laws and the revival of the white nationalist movement over the Confederate flag issue.

An annual report of militias released this week by the Southern Poverty Law Center identified 276 militia groups in the U.S. – a 37 percent increase over the 202 groups identified in 2014.

The current expansion, according to the Law Center, came in the aftermath of a 2014 standoff between federal authorities and hundreds of armed anti-government activists on the property of Nevada rancher Cliven Bundy. The government said Bundy owed $1 million for years of grazing his cattle on federal land. It ended peacefully, but authorities were criticized for not prosecuting those involved in the standoff and not collecting the fees.

Although the Oregon takeover is small in comparison, it was “entirely predictable” based on the outcome in Nevada, the Law Center said.

“We believe these armed extremists have been emboldened by what they saw as a clear victory at the Cliven Bundy ranch and the fact that no one was held accountable for taking up arms against agents of the federal government,” Heidi Beirich, director of the Law Center’s Intelligence Project, said in a statement.