TEXAS, U.S. - A fourth parcel bomb exploded in Travis County, a southwest Austin neighborhood, putting the Texas capital on an edge, and leaving two people injured.
According to the police, who have been closely investigating the series of mysterious and unsolved parcel bombings in the area, Sunday’s incident differed from previous explosions.
Austin Police Chief Brian Manley said, "We're not believing that this was similar to previous ones, as in packages left on doorsteps. But instead this was some type of suspicious package that was left on the side of the road, that detonated and injured these two men.”
The police said that both the men hurt in Sunday's blast are in their 20s and had been biking or walking their bicycles in southwest Austin when the explosion took place.
Both the men are being treated for non-life threatening injuries.
Manley said, "What we do understand now, is that the possibility exists this device was triggered in a different mechanism - that being a tripwire.”
The scene of the explosion on Sunday was attended by police and the FBI, with officials pointing out that the latest explosion comes less than a week after police confirmed that the three previous package explosions were connected.
The three previous package bombs exploded at homes in the Texas capital over a period of ten days, killing two people and injuring two others.
The first blast took place on March 2, killing a 39-year-old African-American man Anthony Stephan.
The package was left at the house of the son of Rev. Freddie Dixon, a pastor at the Wesley United Methodist Church who helped found the Austin Urban League.
Then, on March 12, ten days after the first bombing, two explosions shook the region, killing a 17-year-old African-American male, Draylen Mason and leaving his 41-year-old mother critically injured.
Mason was the grandson of prominent Wesley United members Dr. Norman Mason and Lavonne Mason.
Nelson Linder, the local NAACP chapter president confirmed later that both of those killed in the explosions were African-American members of the same church.
Then, just before noon on March 12, a third bombing critically injured a 75-year-old Hispanic woman, identified as Esperanza Herrera, who was at her mother’s house to care for her at the time.
While the explosions took place as thousands of visitors continued to descend on the city for the South by Southwest music, film and technology festival, the similarities between the packages and the fact that the victims were black and Hispanic prompted police to investigate the bombings as potential hate crimes.
Further, in the incidents, the packages were left overnight on the victims' doorsteps and were not mailed or sent by a delivery service.
The U.S. Postal Service doesn't have a record of delivering the package to the East Austin home where Monday's explosion occurred and there is no record of the packages with private carriers like UPS and FedEx either.
With the explosions grabbing national headlines, panic gripped the area and authorities urged residents to call the police if they receive any packages they aren't expecting.
Residents, spooked by the series of deadly package bombs delivered to homes in Austin cast suspicion on packages delivered to their doorstep and informed authorities.
Police soon confirmed that they were investigating the suspected connection between the incidents, linking it to hate crime seriously.
In the days that followed, Austin police said they received 150 calls from anxious residents about suspicious packages.
On Sunday, after the fourth explosion, police told residents of the neighborhood to stay at home until 10 a.m. (11 a.m. ET) Monday.
Manley said in a late-night news conference on Sunday, “We will not be able to send school buses into the neighborhood on Monday. In addition to that, we're going to ask the residents in the Travis Country neighborhood to stay in your homes tomorrow morning and give us the opportunity to process the scene once the sun comes up.”
Manley has also raised the possibility that a tripwire triggered the device in Travis County.
Meanwhile, he urged the community “to have an extra level of vigilance and pay attention to any suspicious device whether it be a package, a bag, a backpack, anything that looks out of place and do not approach it.”
Manley also said on Sunday that there is still a possibility that the bombings were racially motivated.
He said, "We don't have any evidence. What we know for certain is: We have three victims that are victims of color, and we have three package bombs that have exploded on the east side of Austin," where many of the city's minority residents live.
Adding that investigators believe the two earlier bombings were "meant to send a message.”
Manley added that he hoped the bomber was watching and would "reach out to us before anyone else is injured or killed."
Further, Manley said that local and federal authorities have increased the reward for information leading to a conviction to $100,000.
Texas Governor Greg Abbott was also offering $15,000.
According to Brian Jenkins, an analyst with Rand Corp. who has studied bombings, Manley's invitation to contact authorities could prove fruitful.
He cited the case of the Unabomber, Theodore Kaczynski, who killed three people and injured nearly two dozen more during a bombing campaign that lasted two decades.
He pointed to his "desire to communicate, to have some kind of pronouncement or manifesto."
Jenkins added, “He made the offer that he'd suspend his campaign if his manifesto was published. The publication of that ultimately led to him being identified."
Jenkins said that communication from Muharem Kurbegovic, who was convicted of a bombing that killed three people at Los Angeles International Airport in 1974, helped police narrow their search and apprehend him.
He said, such bombings aren't easy to solve without communication — or without more "events" to provide more clues, adding, “This isn't like a convenience store holdup.”
Jenkins said, “This requires reconnaissance. This requires target selection. They have to think about building a device that works. They have to build that device. They have to think about delivering that device in a way that enables them to conceal their identity."
Jenkins added, “A key question is determining what motivated the bomber or bombers. Were the attacks a one-off event driven by personal grievance — or were they the beginning of something larger? These individuals who become serial bombers — they start campaigns and we don't necessarily understand what their campaigns are. Motives that seem reasonable to them are not discernible to us."
Now, officials have urged residents to call in with tips to the police department, even if the information is seemingly "inconsequential."