Austin, TX – Police said that fourth bomb to explode in Austin showed “a different level of skill” from the package bombs used in the three prior attacks.
The latest detonation happened a couple hours after a press conference where Austin Police Chief Brian Manley urged the bomber to turn themselves in, and more than doubled the reward for information leading to an arrest to $115,000, ABC News reported.
There were a lot of differences between the latest bombing and the three previous attacks, police said.
The first three were package bombs, and were sent to black and Latino recipients. However, on March 19, two white men were injured by a blast that appeared to be set off by a trip wire.
"We've said from the beginning that we're not willing to rule anything out just because when you rule something out you limit your focus," Chief Manley told ABC News. "So this does change the concerns that we had initially although we have still not ruled it out until we understand what the ideology and motive is behind the suspect or suspects."
Police were unable to begin thoroughly investigating Sunday night’s explosion until daylight because of the dangers posed by the potential of other trip wires or booby-trap-type devices.
"There was a description that we may be dealing with a tripwire on this explosive and so out of an abundance of caution we've locked the neighborhood down to keep everybody safe and we're going to go in at sun up this morning and begin processing the post-blast scene," Chief Manley told ABC News.
On Monday morning, residents were told to stay inside their homes, children’s absences were excused from local schools, and anyone who needed to leave the area before police gave the all-clear signal was required to call 911 to get permission and an escort, according to KTBC.
"That concern is legitimate and real," Austin Mayor Steve Adler said, addressing the anxiety of residents in the city.
The mayor said residents should be reassured by the “massive” police response to the bombings, and the “droves” of federal agents who were investigating the explosions with the Austin police, KTBC reported.
Despite the different delivery and victim characteristics, police said they believed Sunday’s explosion was linked to the two package bombs that exploded on March 12, and the first one that blew up on March 2.
"We are working on the belief that this is related to the other bomb incidents that have happened in our community," Chief Manley told ABC News.
A 17-year-old boy was killed in the second bombing, and a 39-year-old man was killed in the March 2 blast.
On March 12, a package bomb exploded at about 6:45 a.m. in a neighborhood on the northeast side of the city, the chief said.
The teenager was killed, and a 40-year-old woman was seriously injured, police said.
At about 11:50 a.m. the same day, police received a call about a second blast nearby.
A package had exploded in the Montopolis neighborhood, southeast of downtown Austin, according to FOX News.
On Monday afternoon, Chief Manley said that in the second incident, a 75-year-old Hispanic woman had opened a package that she found outside her home, and it exploded, too.
The woman was "significantly injured" by the blast and had life-threatening injuries, the chief said.
Anthony Stephan House, 39, was killed on March 2 after a "device" exploded on the front porch of his home in the city's northeast Harris Ridge neighborhood, about 12 miles north of the first March 12 blast, FOX News reported.
The first three explosions took place within 16 miles of each other. Sunday’s blast occurred in a neighborhood just a few miles south of the third explosion scene.
Two of the three explosions occurred in the early-morning hours, police said. Both of those packages were left overnight on the victims' doorsteps, and were not mailed or sent by a delivery service, ABC News reported.