The Austin police chief, Brian Manley, said that an explosion on Sunday night that injured two men was most likely caused by a trip wire and that the device showed "a higher level of sophistication" and skill.
Manley said that they are still not sure if the incidents should be labeled as terrorism, but that they now believe they are dealing with a "serial bomber".
Manley told reporters Monday that investigators say they now believe the person behind the bombings has "a higher level of sophistication, a higher level of skill" than they thought.
It may be related to the three prior explosions, which killed a 39-year-old man and a 17-year-old boy and injured two others, Manley said, but the police chief warned this type of device is even more risky. Both men are expected to recover.
FBI and U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives agents were at the scene, the Federal Bureau of Investigation said on Twitter.
"That concern is legitimate and real", Adler said, adding that residents should also be reassured by the massive police response to the attacks. The prior victims were people of color, which had set off fears that maybe the bombings were racially motivated.
The attack happened in the southwestern Austin neighborhood of Travis Country. Manley asked any neighbors with security camera footage to call police. It could have maimed any passerby, police said.
Two bomb technicians and a bomb canine from SAPD will travel to Austin to help with the investigation of the bombings that have kept Austin on edge. However, officials say Sunday's incident is more sophisticated than the previous ones due to a possible tripwire being involved.
For one, police said the bombing appeared less targeted.
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Residents were told to stay in their homes in the west side neighborhood several miles from where the earlier blasts occurred, Manley said. "Although, we have not yet ruled [the possibility of a hate crime] out until we understand what the ideology or motive is behind the suspect or suspects".
Now, "it's obvious that you have to be alert".
He said the manhunt for the bomber was the "highest priority", with an "army of federal agents" and state resources dedicated to the task. "And that's one of the investigative avenues that we are tracking".
USA code defines domestic terrorism as life-threatening acts that appear meant to intimidate civilians; influence government policy through intimidation or coercion; or affect the conduct of government through mass destruction, assassination or kidnapping. "I think everybody can now say, 'Oh, that's like my neighborhood, '" he said.
Fred Milanowski, agent in charge of the Houston division of ATF, said the latest explosive device was anchored to a metal yard sign near the head of a hiking trail in a residential neighborhood. Two white men in their 20s were injured in the blast but are expected to be OK.
Authorities are working under the belief that the latest incident is connected to the previous three explosions in the city, Manley said. Those packages killed two earlier this month.
Manley confirmed Monday that his department does have persons of interest, but can not identify a suspect at this time.
"He hugged me every morning before class", Austin Community College professor Samuel Osemene said. "And do not approach items like that".
An FBI official on Monday pleaded with whoever is responsible for a series of bombings in Austin, Texas, to come forward.