By Charlie De Mar

CHICAGO (CBS) — Authorities believe that the massive RV explosion Christmas morning in Nashville was intentional. Terrorism experts in Chicago say in the early stages this case has signs of a demonstrative terrorism attack, meaning the person behind it wants attention.

Police in Nashville Friday found tissue that could be human remains among the debris left behind from that early morning explosion. It’s not clear if it belonged to the person behind the attack.

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A warning to leave the area was given before the blast, and the focus has now shifted to a motive.

“There is usually a video post. There’s usually a blog post. There’s usually some sort of written communication left behind,” said Robert Pape, a professor and director of the Chicago Project on Security and Threats at the University of Chicago.

A motive could be revealed in the coming hours, according to Pape, who is known best for studying suicide terrorism.

“Motive comes directly from the perpetrator. There’s no secret way that we have to track motive,” he said. “I would say that it is most likely demonstrative terrorism. That is an act trying to threaten harm to others to bring attention to a political cause where the goal was to create maximum attention, not to kill the maximum number of people.”

Police say the blast was intentional and achieved its goal of creating chaos.

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“There are several factors that show you that the perpetrator went to efforts to minimize the damage,” Pape said.

Those include early, empty streets and Christmas morning.

More than 40 buildings were destroyed. Three people taken to hospitals with non-life threatening injuries. Police spent much of the day going through the damage.

“If the police really thought there was a perpetrator running loose through the city, this would turn into a man hun a la Boston bombing,” Pape said.

Federal and local police are promising to find out who is behind the explosion that rocked Christmas morning.

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Chicago police are monitoring the situation but Friday night said there was no credible threat to the city.

Charlie De Mar