The latest blast occurred just after 8:30 p.m. local time Sunday. Two bicyclists, white men aged 22 and 23, suffered injuries described as serious but not life-threatening after setting off a tripwire attached to a bomb in a Travis County neighborhood in southwestern Austin.
After the blast, authorities asked residents of the neighborhood not to leave their homes until 2 p.m. local time Monday.
The latest explosion marks a departure from the first three blasts, which were left at people's homes. Authorities also noted Monday that the explosive devices are getting more sophisticated.
"We are clearly dealing with a serial bomber," Austin Police Chief Brian Manley said in a Monday press conference.
"We will have to determine if we see a specific ideology behind this."
"We now need the community to have an extra level of vigilance and pay attention to any suspicious device — whether it be a package or a bag, a backpack — anything that looks out of place. And do not approach items like that," Brian Manley added.
The FBI has dispatched 350 special agents to Austin since the Sunday explosion.
"With this tripwire, this changes things," said Christopher Combs, special agent in charge of the FBI's San Antonio division. "It's more sophisticated, it's not targeted to individuals… a child could be walking down a sidewalk and hit something."
In the first week of March, 39-year-old Anthony Stephen House was killed when a package exploded in his home. Authorities at the time described it as an "isolated incident."
March 12, two other package explosions took place: the first killed 17-year-old student Draylen Mason while the second explosion critically injured 75-year-old Esperanza Herrera, who was visiting her mother at the time of the incident. The bomb that injured Herrera was not addressed to her home. It is unclear whether she opened a box not addressed to her.
House and Mason, who are black, had relatives who were reportedly good friends and important members of Ausin's black community. Freddie Dixon, House's stepfather, told the Washington Post that he is good friends with Mason's grandfather, Norman Mason. They were fraternity brothers and Norman Mason went to a church where Dixon used to be a pastor.
In all three instances, the packages were found outside the homes in the early morning and were not delivered by any mail services. The similarities between the packages and the fact that the first three victims were minorities impelled police to investigate the bombings as potential hate crimes. It is not known whether the fact that the latest victims are white will change the direction of the investigation.
Austin police are offering a $100,000 reward for information leading to the arrest of the person or people responsible for the bombings.
On Saturday, Austin police arrested 26-year-old Trevor Weldon Ingram for making a bomb threat at the South by Southwest festival, targeting a venue where popular hip-hop band The Roots was scheduled to perform that night.
The emailed threat was reported to police by a Live Nation Music representative. The concert's sponsor, Bud Light, canceled the concert even after officials found no explosives at the site.