Austin Police Suspect a 'Serial Bomber' is Attacking City After Fourth Explosion

After a fourth explosion in Austin, Texas, Sunday, police said they think they are dealing with the same person repeatedly attacking the city.

"We are clearly dealing with a serial bomber," said Austin Police Chief Brian Manley, according to NBC News.

The latest explosion was apparently triggered by a tripwire and injured two people on bicycles Sunday, while the previous attacks were explosions in packages.

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"We are working under the belief that this is related to the other bombing incidents that have occurred in our community over the last couple weeks," said Manley, according to The Washington Post.

Two people have died from the multiple bombings—a 39-year-old man and 17-year-old boy—both of whom were black and related to prominent people in the African-American community in the city. Some speculated the attacks could be race-related. The most recent explosion, however, seems more random, authorities said.

"With this tripwire, this changes things," said Christopher Combs, an FBI special agent at the bureau's San Antonio division, according to NBC News. "It's more sophisticated, it's not targeted to individuals ... a child could be walking down a sidewalk and hit something."

Police did not, however, say they could yet designate the bombings as terrorism. According to CNN, think tank Stratfor Threat Lens speculated the tripwire indicated the attacker could have training. 

"The device's success, despite significantly different design, further suggests that the bombmaker behind these attacks is an accomplished one, and has likely to have received some training, perhaps as a military or police explosive ordnance disposal technician," the group said. 

Police are still figuring out if the explosions were hate crimes as they work to determine who is behind the attacks.

"We've said from the beginning that we're not willing to rule anything out, just because when you rule something out you limit your focus," Manley said in an interview Monday with Good Morning America. "This does change the concerns that we had initially, although we have still not yet ruled it out until we understand what the ideology or motive is behind the suspect or suspects."