The capital of Texas is on edge again after two more people were injured in another explosion there Sunday night.
The latest blast occurred around 8:30 p.m. in a suburban neighborhood in southwest Austin.
Two men in their 20's were hurt and hospitalized with serious but non-life-threatening injuries.
It was the fourth explosion to rock Austin in less than three weeks and police say the bombings are likely related. Police Chief Brian Manley says investigators see "similarities" in the bombings but the latest attack is more sophisticated.
Police are warning residents not to touch any unexpected packages left at their homes. They say the latest explosion was detonated by a tripwire, "showing a different level of skill."
Austin Mayor Steve Adler wants residents to be on the lookout and call 911 right away if they see anything suspicious. He says concern is growing because "the methodology has changed."
Officials had been investigating the incidents as hate crimes since all the initial victims were black or Hispanic. But the two latest victims were white, so police now say they're not ruling out any possible motive for the bombings.
And police are even reaching out directly to the terrorist, asking the bomber to communicate with them and tell them why he or she is doing it.
The first bombing took place on March 2, killing a 39-year-old man. Two more package bombs then exploded in front of homes on March 12, killing a 17-year-old, wounding his mother and injuring a 75-year-old woman.
Authorities are offering a $115,000 reward for information leading to the arrest of the bomber.