OKLAHOMA CITY, Oklahoma — The United States will mark the 15th anniversary of its deadliest domestic terrorism attack on Monday amid rising political tensions and anti-government sentiment that has terrorism experts on edge.

The April 19, 1995, truck bombing of the Alfred P. Murrah federal building in downtown Oklahoma City by members of an anti-government militia killed 168 people, including 19 children, and injured hundreds of others.

Former President Bill Clinton, who oversaw the recovery efforts and investigation, warned that there are frightening parallels between the current political tensions and those of the years leading up to the bombing.

He warned there could be consequences of "serious demonizing the government and its employees and a whole effort to legitimize violence."

"We can't let the debate veer so far into hatred that we lose focus of our common humanity," Clinton said in a speech Friday at the Center for American Progress.

There has been a dramatic growth of hate groups and anti-government 'patriot' groups in the wake of a deep economic downturn and the election of the first black president, said Heidi Beirich, director of research for the Southern Poverty Law Center.

The number of active "Patriot" groups nearly tripled last year to 512 from 149 in 2008, according to the Southern Poverty Law Center, which tracks the activities of hate groups.

Some 127 of those groups were paramilitary militias, up from 42 a year earlier.

The total number of active hate groups grew to a new high of 932 last year. The previous high was 858 in 1996.

"We saw a massive expansion," Beirich said in a telephone interview. "And we don't expect much of a letdown."

One notable case is that of nine members of a radical Michigan-based "Christian warrior" militia charged last month with plotting to kill police then bomb the victim's funeral in a bid to unleash an uprising against the US government.

A terrorism expert said the national mood is ripe for the resurgence of the militia-type groups similar to those that attracted Timothy McVeigh, who was convicted and executed for detonating explosives parked a van outside the federal building.

"What we're missing right now is the symbolic catalyst," said Dr. Brent Smith, director of the Terrorism Research Center at the University of Arkansas.

"There has always been some significant event that pushed the extreme right over the edge."

Two catalysts for the rise of the militia movement in the 1990's were a violent 1992 siege by federal officials at the Ruby Ridge home of a radical Idaho family and the 1993 FBI standoff in Waco, Texas with a Branch Davidian religious sect which left 76 people, including 20 children, dead.

Smith said the federal government learned from its mistakes and responds to such incidents with more restraint but cautioned that the threat is rising despite heightened vigilance in the wake of the September 11, 2001 Al-Qaeda attacks that killed 2,973 people.

"We can't harden everything and there are plenty of opportunities for folks of this nature," he said. "It depends on how the government addresses these issues. I think the threat can be managed."

Kari Watkins, executive director of the Oklahoma City National Memorial and Museum, said lessons learned from the Oklahoma City bombing are as relevant today as they were 15 years ago.

"Americans need to decide to come together and unite instead of letting people and situations divide us," Watkins told AFP.

"I hope that people will remember the unity this nation had following the Oklahoma City bombing and work to replicate it. There is nothing that can justify what happened in Oklahoma City and the senselessness of terrorism and violence."

The 15th anniversary of the Oklahoma City bombing will be recognized at the Oklahoma City National Memorial beginning at 8:55 am Monday.