Bob Daemmrich for The Texas Tribune
For the fourth time this month, a bomb exploded in an Austin residential neighborhood Sunday night, seriously injuring two people and alarming law enforcement that a sophisticated “serial bomber” might be at work in Texas' capital city.
The most recent explosion appears to have been triggered by a tripwire mechanism, police said, signifying a level of sophistication and perhaps randomness that hadn’t been seen in the previous bombings, police said.
Still, authorities said there are similarities in the case to lead investigators to believe that the bombings are related.
"We are clearly dealing with what we expect to be a serial bomber," said Brian Manley, Austin's interim police chief.
The most recent bombing happened in an upper-middle-class neighborhood in the southwest portion of Austin. Manley said that a 22-year-old man and a 23-year-old man were walking along a sidewalk when they inadvertently triggered the bomb, which was sitting near a fence.
Both men were seriously injured and remained hospitalized late Monday morning. Police said their conditions were stable.
That raises the total number of victims in the bombings to six – two who were killed and four others who were seriously injured. But several things were different in the most recent attack. For one, police said the bombing appeared less targeted. In the prior three cases, the bombs appeared to be packages left at specific houses.
In addition, the victims of the most recent attacks were white. The prior victims were people of color, which had set off fears that the bombings might be racially motivated.
This was also the first bombing outside of East Austin, an area of the city that has historically been home to black and Hispanic residents.
With the change in methods and the new location of the most recent bombing, authorities said they were at a loss over the motive of the attacks.
“We don’t know why the bomber is doing this, we don’t know his reasons,” said Christopher Combs, special agent in charge of the San Antonio field office.
Hours before the fourth bombing, Manley made a public request for the perpetrator to reach out to police and explain the “message” of the attacks. He made a similar plea Monday.
“Communicate with us so we can put this to an end,” he said. “There are innocent people getting hurt in this community.”
Manley also urged residents to be even more cautious than they have been before. Anyone could have been hurt in the most recent bombing, even children, he said.
"The belief that we are dealing with someone who is using trip wires shows a higher level of sophistication, a higher level of skill," Manley said.
The two people killed in prior bombings – 39-year-old Anthony Stephan House and 17-year-old Draylen Mason – died on March 2 and March 12, respectively. Mason's mother was also injured.
Hours after the explosion that killed Mason, 75-year-old Esperanza Herrera also received a exploding package bomb. She was seriously injured but is expected to survive.
Hundreds of law enforcement officers have responded to the attacks. In addition to Austin police, more than 350 FBI agents are working the case along with Texas Department of Public Safety troopers and bomb technicians from Houston and San Antonio.
On Sunday, Manley announced a reward of $100,000 for information leading to the arrest of the person or people responsible. Gov. Greg Abbott previously said that the Office of the Governor’s Criminal Justice Division is offering an additional $15,000.
"First and foremost, Cecilia and I offer our thoughts and prayers to the victims of these atrocious attacks," Abbott said. "I want to assure all Texans, and especially those in Austin, that local, state and federal law enforcement officials are working diligently to find those responsible for these heinous crimes."