Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of " United States of Jihad: Investigating America's Homegrown Terrorists ." David Sterman is a senior program associate at New America's International Security Program and holds a master's degree from Georgetown University's Center for Security Studies.

(CNN) Thursday's attack in Dallas provides a reminder that violent extremism and terrorist attacks are not the province of any one ideology. Instead, political violence has a long history in the United States and not just from people drawn to Osama bin Laden and his successors' vision of global jihad.

The suspected Dallas sniper who killed five police officers has been identified as Micah Xavier Johnson, a 25-year-old from Mesquite, Texas.

The Dallas shootings, like the one carried out last year by Dylann Roof on a black church in Charleston, South Carolina, that left nine people dead, were politically motivated.

Roof said he wanted to start a "race war," officials said, and he posted racist screeds on a white supremacist site.

Johnson "wanted to kill officers, and he expressed killing white people, he expressed killing white officers, he expressed anger for Black Lives Matter," Dallas police Chief David Brown said.

On his Facebook page, Johnson poses with a clenched fist as if delivering a Black Power salute.

Terrorism is generally understood to be acts of violence conducted against civilians for political purposes. Killing white police officers who are guarding a peaceful demonstration certainly qualifies as terrorism, in the same way that Roof's attack on black churchgoers does.

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The Dallas attack is the first instance of deadly terrorism seemingly motivated by extreme left-wing ideology in the post-9/11 era. Brown called it a "a well-planned, well-thought out, evil tragedy."

There have been anti-police attacks in which the perpetrators linked their actions to jihadist ideology. In October 2014, Zale Thompson, 32, who, police said became radicalized by reading ISIS-related material, attacked officers in Queens, New York, with a hatchet, critically injuring one of them. He was shot to death by police.

In January, Edward Archer, a 30-year-old, shot and wounded a police officer in Philadelphia, and authorities said he told them he did it for ISIS.

There have also been deadly anti-police terrorist attacks by far-right militants. On June 8, 2014, Jerad Miller and Amanda Miller killed two police officers in an ambush at a pizza restaurant in Las Vegas before committing suicide. The couple left a note referencing revolution and Jerad Miller had a history of anti-government posts online and had traveled to Cliven Bundy's ranch in Nevada during the early 2014 standoff between armed ranchers allied with anti-government militias and the police.

But there hasn't been a case of lethal terrorism emanating from the left for more than a decade and a half. In the post-9/11 era, left-wing plots have tended to target property. The FBI considers militant animal rights and eco-terrorism groups as a top domestic terrorism priority, though their violence has resulted in no deaths.

During the 1970s, terrorist attacks by leftists were far more common. The Black Panthers and their splinter groups carried out a number of bombings and assaults. So too did the Weather Underground and its splinter groups.

These leftist militant groups largely disappeared in the 1980s, and since then lethal domestic terrorist attacks, such as the 1995 bombing of the federal building in Oklahoma City, have either been carried out by far-right or by jihadist terrorists, as we saw last month in Orlando and in December in San Bernardino, California.

Now law enforcement must focus, once again, on the possibility that far-left militants may carry out lethal attacks.