Police told residents of a neighborhood in southwest Austin to stay at home until 10 a.m. (11 a.m. ET) Monday after the fourth explosion in less than a month hit Texas' capital, injuring two men.
In a late-night news conference on Sunday, Austin Police Chief Brian Manley raised the possibility that a tripwire triggered the device in Travis County.
“We will not be able to send school buses into the neighborhood on Monday,” he said. “In addition to that, we're going to ask the residents in the Travis County neighborhood to stay in your homes tomorrow morning and give us the opportunity to process the scene once the sun comes up.”
The men hurt in Sunday's blast — both in their 20s — were being treated for non-life threatening injuries, officials said.
Manley asked 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.”
Police are working under the belief that the incident is related to a string of unsolved package bombings this month which killed two and injured two others, though that has not yet been confirmed.
Stephen House, 39, was killed on the morning of March 2, and Draylen Mason, 17, died on the morning of March 12. Both were African-American members of the same church, Nelson Linder, the local NAACP chapter president, told NBC News last week.
The men hurt in Sunday's blast — both in their 20s — were being treated for non-life threatening injuries, officials said.
Manley asked 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.”
Police are working under the belief that the incident is related to a string of unsolved package bombings this month which killed two and injured two others, though that has not yet been confirmed.
Stephen House, 39, was killed on the morning of March 2, and Draylen Mason, 17, died on the morning of March 12. Both were African-American members of the same church, Nelson Linder, the local NAACP chapter president, told NBC new last week.
Mason's 41-year-old mother was also critically injured in the explosion.
Then, just before noon on March 12, a third bombing critically injured a 75-year-old Hispanic woman, Esperanza Herrera.
Linder, of the NAACP, added that someone connected to the House or Mason families was the intended target in the third explosion, although he declined to provide additional details.
Asked Sunday whether the bombings were racially motivated, Manley said it was possible.
Police believe the two earlier bombings were "meant to send a message," though Manley didn't say what that message was during a news conference earlier Sunday.
The fourth explosion came just hours after officials made a direct appeal to the person responsible to contact them.
In that press conference, Manley said that he hoped the bomber was watching and would "reach out to us before anyone else is injured or killed."
The plea came as local and federal authorities increased the reward for information leading to a conviction to $100,000, Manley said. Texas Gov. Greg Abbott was also offering $15,000.
"We don't have any evidence," he said. "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.
NBC News analyst and former FBI profiler Clint Van Zandt said while it's not clear whether this latest package explosion was done by the same person responsible for the previous ones, the fact that this one used a tripwire implies the methods used are "evolving."
"This is something we see with serial offenders, serial killers, serial bombers, serial arsonists. Unfortunately they learn as they go," Van Zandt said. "And they're wondering what police know so they can change."
Brian Jenkins, an analyst with Rand Corp. who has studied bombings, said in an interview that Manley's invitation to contact authorities could prove fruitful.
He pointed to the Unabomber, Theodore Kaczynski, who killed three people and injured nearly two dozen more during a bombing campaign that lasted two decades, and his "desire to communicate, to have some kind of pronouncement or manifesto."
"He made the offer that he'd suspend his campaign if his manifesto was published," Jenkins said. "The publication of that ultimately led to him being identified."
Such bombings aren't easy to solve without communication — or without more "events" to provide more clues, Jenkins said.
"This isn't like a convenience store holdup," he said.
"This requires reconnaissance," Jenkins added. "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."
A key question, Jenkins said, is determining what motivated the bomber or bombers.
"These individuals who become serial bombers — they start campaigns and we don't necessarily understand what their campaigns are," he said. "Motives that seem reasonable to them are not discernible to us."
In 2002, for instance, college student-turned-suspected-pipe-bomber Lucas Helder planted bombs in mailboxes across the United States in an arrangement that would allow someone looking at the United States from space to see a smiley face.
"Those are things that are not easy for outsiders to figure out," Jenkins said.
