The most recent bomb may have been detonated by a roadway trip wire, cops said Sunday.
The city of Austin is on high alert after a fourth explosive detonated in the city in just a matter of days.
Police on Sunday said a bomb left on the side of a road exploded, injuring two people. Investigators said it may have been detonated by a trip wire.
"It is very possible that this device was a device that was activated by someone either handling, kicking, or coming in contact with a tripwire that activated the device," said Austin Interim Police Chief Brian Manley.
The victims were hospitalized in good condition, though one reportedly suffered an apparent shrapnel wound that was caused by a nail.
The blast rang out in the Texas capital not long after authorities there announced a total reward of $115,000 was being offered for information leading to the arrest and conviction of whoever is responsible for three previous package bombs, which left two dead.
The fatally wounded victims, 39-year-old Anthony House and 17-year-old Draylen Mason, and two others who were injured in the earlier bombings were all black or Hispanic. The identities of Sunday night's victims were not immediately known.
Chief Manley told reporters he believes the bombs were meant to send a message.
"We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed," Chief Manley said, per CBS News.
Residents in the area of the explosion Sunday were warned to stay indoors following the explosion and that warning extended into Monday.
"We now need the community to have an extra level of vigilance and pay attention to any suspicious device, whether it be a package, a bag, [or] a backpack," Manley told reporters as local officials, the FBI and ATF waited for daylight to come before confirming the neighborhood was safe.
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