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Another bombing in Austin injured two men. Police officers talk to commuters as they work the scene of a bombing on March 19, 2018, near the 4800 block of Dawn Song Drive in Austin, Texas. SUZANNE CORDEIRO/AFP/Getty Images

Two men were injured Sunday night in a roadside bomb in Austin, Texas, in the latest of four explosions that have struck the city in March. Authorities say the latest bombing had "similarities" with the other three, leading them to believe they are the work of a serial bomber.

Police responded to an explosion that was accidentally detonated by the two men as they rode past it on their bicycles. The Austin Chronicle reported that the two men are in stable condition.

Like the other three incidents, this was a package bomb. But unlike the other bombings, the latest bombing was left closer to the street, rather than on a front doorstep. The bomb may have been triggered by a tripwire, leading Austin Police Chief Brian Manley to suspect a high level of technical prowess by the bomber.

Manley also said police believe this most recent bombing was related to the previous ones. He urged neighborhood residents to stay inside until late Monday morning so authorities could secure the area and ensure their safety.

The first Austin bombing occurred on the morning of March 2, when a man was killed by an exploding package he picked up on his front porch. Two separate package bombs were detonated on March 12, killing a male teenager and injuring two women.

Austin police offered a $100,000 reward for information leading to the arrest of the bomber, while Gov. Greg Abbott will provide an additional $15,000.

Manley stated that the bombs were "meant to send a message," though the motives remain unclear.

"We want to understand what brought you to this point, and we want to listen to you,” Manley said in an appeal for the bomber to come forward.

Prior to Sunday’s attack, a prevailing theory was that the bombings could possibly be racially motivated hate crimes. The first three explosions took place in predominantly non-white neighborhoods, targeting exclusively black and Hispanic residents. However, Sunday’s incident happened in a neighborhood further to the west, and both victims were white men.

If Sunday’s bombing was indeed carried out by the same perpetrator as the previous explosions, it would represent a slight change in tactics. According to police, whoever is doing this is leaving the packages out overnight, not sending them through official delivery services, according to the BBC.

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Another bombing in Austin injured two men. Police officers talk to commuters as they work the scene of a bombing on March 19, 2018, near the 4800 block of Dawn Song Drive in Austin, Texas. SUZANNE CORDEIRO/AFP/Getty Images