The U.S. Department of Justice says it is investigating the killings of nine black worshippers in Charleston, S.C., as a potential domestic terrorism crime.

The announcement on Friday came amid a growing chorus of voices calling on government officials, law enforcement and mainstream media to label the crime as what it really is – an act of terrorism.

The Charleston murders have renewed a long-simmering debate over why some incidents are called terrorism and others are not – and why the term "terrorist" is largely reserved for Muslims and Arabs.

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"This fits the definition of terrorism and shows all the hallmarks of what we know as far as the tactics that domestic terrorists use," said Daryl Johnson, who worked at the U.S. Department of Homeland Security from 2004 until 2010 and now runs a security consultancy, DT Analytics.

Terrorism is a debated term, he said, but the key criteria are generally that an act is politically or socially motivated, and meant to instill fear or a change in government policy.

Reports of the alleged shooter's talk of starting a "race war" suggest exposure to white supremacist ideas and a larger agenda, he said, adding that the choice of a symbolic target – in this case, one of the oldest African-American churches in the United States – is a tactic terrorists use.

"He selected this target because it has symbolic value within the African-American community and the history that it ties to the civil rights movement and other things," he said.

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Black writers questioned officials' reluctance to use the term "terrorism."

"The violence that's been committed against black people in the U.S. including and since slavery is one long, politically motivated act of terror," argued Mic writer Zak Cheney-Rice.

Read more: Aftermath of Charleston: Five black American writers you should read today

In the online and social media reaction to the Charleston tragedy, Lisa Stampnitzky, a Harvard University lecturer and author of Disciplining Terror: How Experts Invented "Terrorism," sees many ordinary Americans – especially African Americans – trying to apply the label to an attack they believe was meant to induce terror among black people.

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"My sense is that the motivation to label this attack terrorism is that it gives more gravity to the attack, and one reason to do that is violence against African Americans in the U.S. has not been treated as seriously as violence against white Americans," she said.

"This is part of a movement to bring weight and show that black lives matter," she added.

Charles Kurzman, a professor of sociology at the University of North Carolina and an expert on terrorism, is not surprised by the online and social media debate. "These issues are very real for people when you have an event that is a lynching like the shootings in Charleston, and emotions run high," he said.

But he does not think consensus will ever be reached on all incidents that could be terrorism, and he is in no rush to decide on the Charleston killings. He said he wants to find out more about the alleged shooter's motivations and political ideology, and to see whether the killings were intended as an act of spectacular politically motivated violence. He also wants to find out whether the perpetrator considers his actions terrorism, and if he has any ties to organization and ideological movements.

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The public is unaware of the challenges getting a conviction of terrorism, he said. "It is easier to convict someone on pre-mediated murder and the penalty is the same," he said, adding that terrorism charges involve the need to show political motivation.

Stephen Toope, director of the Munk School of Global Affairs, sees the calls to label events in Charleston as terrorism as part of a broader trend since the 9/11 attacks to expand the definition of terrorism.

"I think in some ways that's been quite unhelpful, because it's allowed for claims of the need to address terrorist attacks that previously would not have been conceptualized as terrorism," he said.