>>>As events continue to unfold in connection with Monday’s terrorist attack in Boston, Americans are marking some other somber anniversaries that fall just days after that attack. April 19 is the anniversary of the 1993 Branch Davidian siege in Waco, Texas, and the 1995 terrorist attack on the federal building in Oklahoma City.

They are tragic images seared into America’s collective memory, and both took place on April 19.

The 1993 siege at the Branch Davidian compound in Texas and resulting fire killed more than 70 members of a radical religious sect after a confrontation with federal officers.

Two years later, a devastating truck bomb ripped apart the federal building in Oklahoma City, killing 168 people and injuring hundreds of others.

Former soldier Timothy McVeigh was convicted of that attack and was executed in 2001.

Since then, the date of April 19 has taken on special significance, according to Mark Potok of the Southern Poverty Law Center. He monitors radical right wing groups in the U.S. and spoke to VOA via Skype.

“Well April 19th has become a kind of iconic date in the radical right in the United States," said Potok. "That really begins because that is the day in 1775 when the opening shots of the American Revolution are fired, of course, in Lexington and Concord.”

Another notable mid-April tragedy was the high school shooting in Columbine, Colorado, on April 20, 1999.

The anniversaries are often a time for sad reflection by Americans, but also a source of concern for law enforcement.

Former federal prosecutor Aitan Goelman was on the team that tried the Oklahoma City bombing case. He recalled the impact of the bombing on the American public.

“It was a real shock to the system, and the fact that it took place in Oklahoma City, in the heartland, and that it turned out not to be a Muslim fanatic but a homegrown American terrorist, I think, shook people up a great deal,” he said.

And Goelman said there is a direct linkage between the Waco standoff and the Oklahoma City attack carried out by McVeigh.

“He viewed Oklahoma City as payback for Waco," he said. "And the date, April 19, that was not coincidence. McVeigh deliberately chose that date because it was the Waco anniversary.”

The Oklahoma City case highlighted McVeigh’s links with right wing anti-government militia groups.

Those groups began to fade in the late 1990’s but are now making a comeback, said Mark Potok.

“Until 2009, immediately after Barack Obama’s election, [is] when we saw absolutely tremendous growth," he said. "There were 149 of these groups by our count in the year 2008. By last year, 2012, that number had reached 1,360.”

The recent focus in Washington on gun control following the Connecticut school shooting is also stirring passions among extremist groups on the right, said former prosecutor Aitan Goelman.

“Gun control is a hot button issue for the lunatic fringe," he said. "They see any even modest step toward controlling guns as the slippery slope, the beginning of the end to when some new world order or some U.N.-backed federal government is going to come in and pry everybody’s guns out of their hands.”

And so even as Boston copes in the aftermath of this latest terror attack, April 19 brings painful reminders of past tragedies and the need to be prepared for future threats, both foreign and domestic.