Police said Sunday night's explosion was connected to three previous bombings in Austin and the bomber used a tripwire in the latest attack, showing a "higher level of skill."
"We're clearly dealing with what we expect to be a serial bomber at this point," Austin police chief Brian Manley told reporters.
But Manley said police have been unable to determine a motive for the bombings that have killed two people in the city of nearly one million people and injured another four.
"Is this terrorism? Is it hate-related?" Manley asked. "As we said from the very beginning, we were not willing to classify this as terrorism, as hate, because we just don't know enough."
In the earlier bombings, two African American men were killed by packages left on their doorsteps, raising the possibility of a racially motivated crime. A 75-year-old Hispanic woman was also injured in a blast.
Explosion in Austin wounds two men
But Manley said the latest bombing seriously wounded two white men aged 22 and 23 as they walked along a sidewalk or on the road in a quiet residential neighborhood of southwest Austin.
He said it appeared "random" and was triggered by a tripwire.
"What we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night an attack that would have hit a random victim that happened to walk by," the police chief said.
"We've definitely seen a change in the method that this suspect or suspects are using."
Manley said the use of a tripwire also means police are dealing with someone who "shows a higher level of sophistication, a higher level of skill" than initially believed.
"With this tripwire, this changes things," agreed FBI special agent Christopher Combs. "It's more sophisticated, it's not targeted towards individuals.
"We're very concerned that with tripwires a child could be walking down a sidewalk and hit something."
Manley appealed to the bomber to contact the authorities and to the public to report anything suspicious.
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"I will reach out to the suspect or suspects and ask that you contact us, ask that you reach out to us, communicate with us so that we can put this to an end," Manley said. "There are innocent people getting hurt in this community and it needs to come to a stop."
The authorities said they were increasing the reward offered for information leading to an arrest, bringing the total city and state bounty money to $115,000.
"We need every tip, every piece of information, however inconsequential you may think it is," Manley said.
An exploding package killed 39-year-old Anthony House on March 2. A 17-year-old African American teenager, Draylen Mason, was killed on March 12 and the Hispanic woman was critically injured the same day.
All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores.
Three Democratic members of Congress called for the bombings to be classified as "terrorist attacks" and "investigated as such."
"Also, we need to understand if these attacks are ideologically or racially motivated," Representatives Bennie Thompson, Cedric Richmond and Sheila Jackson Lee said in a statement.
"The community impacted is now under virtual house arrest and the entire city is posed to be in a state of fear — which can easily transition into panic."
A task force of hundreds of members of law enforcement are working the case, including criminal profilers and experts from the Federal Bureau of Investigation and the Bureau of Alcohol, Tobacco and Firearms (ATF).
Austin Mayor Steve Adler said the manhunt was the "highest priority."
"There's an army of federal agents," Adler said on ABC's "Good Morning America." "We have state resources.
"We're going to find out who is responsible for this and we're going to stop it," he said.
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