The chief asked residents in Austin's Travis Country neighborhood to turn in any surveillance footage from any security cameras on their homes.
Mr Manley said the blast involved a tripwire, unlike the first three attacks, in which package bombs were left on people's doorsteps. Manley said they believe the device was activated by a tripwire as the two victims were walking alongside the road.
The explosion wounded two men, who were described by Manley as 22 and 23 year old Anglo men.
All three victims were minorities-Mason and House had ties to prominent members of Austin's African-American community-and police said that they were not ruling out that the bombings were hate crimes.
"The person is actually trying to provoke fear and that is made worse by simply not giving information about why", said Robert Pape, an global security expert and director of the University of Chicago Project on Security and Threats.
Police also said they have been unable to determine a motive for the string of bombings which have killed two people in Austin and put the city of almost one million on edge.
A reward of $115,000 has been offered for information leading to the arrest and conviction of the person or persons responsible. Manley said they are now in stable condition.
"We are clearly dealing with a serial bomber", Austin Police Chief Brian Manley said, adding that he was unable to answer questions from reporters about whether the incidents should be considered domestic terrorism.
In a press conference Monday morning, Manley said a tripwire mechanism appears to have been used in the most recent blast.
Meanwhile, the Federal Bureau of Investigation and Texas Crime Stoppers are jointly offering a $115,000 reward for information leading to the arrest of the serial bomber.
Mayor Steve Adler said the latest explosion would only raise anxieties in the city.
The neighborhood was being locked down until 2 p.m. local time out of an "overabundance of caution", he said, citing "evidence thrown across quite a significant distance" by the roadside bomb.
"So we've definitely seen a change in the method that this suspect or suspects is using", Manley said. "We're one of the safest cities in the country now dealing with some pretty horrific events and I think the important message right now is just to tell people if you see anything that is in anyway suspicious just don't go near it and call 911". "We will have to determine if we see a specific ideology behind this". She was in bed watching TV when the incident first happened, she said, but soon her phone "started blowing up" with text from friends who had heard the news.
He said investigators have yet to establish a motive.
"There's a message behind what's happening in our community", he said, "and we're not going to understand that until the suspect or suspects reaches out to us to talk to us about what that message was".
The Austin Police Department is also offering up to $100,000 for more information.
"We're very concerned that with tripwires, a child could be walking down a sidewalk and hit something", Combs said.
Those bombs were in packages, typically left on doorsteps early in the morning or overnight, officials previously said.
"There are probably more law enforcement officers per capita in Austin than there has ever been before", Gov. Abbott reported".
He then said he wanted to tap into the psyche of the bomber or bombers. Two separate explosions, 10 days apart, killed Stephen House, 39, and Draylen Mason, 17.
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