Adding to the confusion is the fact that Sunday night's two victims - men in their 20s either pushing or riding bicycles - were white, but a racial motive is still possible, the chief said.
"This terrorist waits for the cover of darkness", Gallagher said.
"The kind of person that does this is listening and watching the news", he said.
Four bombs have shaken the city this month.
"It sounded like a very nearby cannon", Holmes said.
Police have received more than 735 calls about suspicious packages since the three parcel bomb attacks, but authorities had not found any that posed a security risk, Manley said.
He said the technicians and canine will assist Austin police and the Federal Bureau of Investigation as long as they're needed.
On Monday morning, he warned: "We now need the community to have extra level of vigilance and pay attention to any suspicious device - whether it be a package or a bag, a backpack - anything that looks out of place".
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. Nelson Linder, president of the Austin chapter of the NAACP, says he's been involved with the investigation from the start, and he's certain the victims were targeted.
"I'm honestly scared to walk anywhere around Austin now", Andres Vecchio wrote on Twitter. Numerous attacks have been solved by the authorities; they often stemmed from domestic disputes, and sometimes involved pipe bombs in packages wrapped like holiday presents.
Bryce Dubee, an Iraq and Afghanistan army war veteran, likened the feeling to his fear of IEDs. "That's what made me feel sick".
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Republican leaders, meanwhile, have been coalescing behind Dutchess County Executive Marc Molinaro as a candidate, with state Sen. Miranda Hobbes for Governor? However, given his close connections to Nixon, de Blasio is unlikely to support Cuomo this time.
While authorities are unwilling to classify the attacks as domestic terrorism or hate crimes, they have not yet ruled out the possibility of either.
That is to say, the newest explosive device contains similarities to the other three bombs, but there are some important differences that suggest a more advanced approach.
Police Chief Brian Manley also told ABC's "Good Morning America" on Monday that both men who were injured in Sunday night's explosion in the southwestern Austin neighborhood of Travis Country are white, unlike the victims in the three earlier attacks, who were black or Hispanic.
Gov. Greg Abbott said today that his office is allocating $265,500 for APD and the Texas Ranger Bomb Response Team to buy X-ray devices that would "provide clear visual evidence" for law enforcement responding to package-related calls.
"Being in a different part of the city now, I think people have gotten the message now that no one is safe and that no matter what your color is", he said.
Police are working under the belief that the explosions are related.
Authorities have cordoned off the neighborhood where the bomb went off Sunday night and have warned residents to remain indoors while officers check for anything suspicious.
"Communicate with us so that we can put this to an end", Manley said.
The coalition organized a town hall last week to bring together community members and law enforcement after the first three bombings.
Police asked for the public's help to catch the "serial bomber".
Authorities maintain a cordon near the site of an incident reported as an explosion in southwest Austin, Texas, U.S. March 18, 2018.
Austin Mayor Steve Adler declined to say if the increased reward possibly prompted the latest explosion because "there's not enough data to know that".