The area around the explosion site has been blocked off and authorities are interviewing neighbors and searching for possible witnesses.
"Any tension that's put on that wire" will activate it, Frederick Milanowski, the special agent in charge for the Houston division of the Bureau of Alcohol, Tobacco and Firearms (ATF), said at a news conference Monday.
Preliminary evidence shows that there are similarities between Sunday night's explosion and three other explosions that Austin has suffered in the past month.
"We believe that the recent explosive incidents that have occurred in the city of Austin were meant to send a message", Austin Police Chief Brian Manley said. Manley said that a 22-year-old man and a 23-year-old man were walking near a fence when they inadvertently set off the bomb.
According to the New York Times, two of the people who were killed belonged to African-American families with deep roots to the city's Black, religious and civil-rights groups.
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Austin Mayor Steve Adler emphasized that the threat is not relegated to packages and reiterated that an "army" of law enforcement is in Austin working on this case.
The latest explosion comes less than a week after police said three previous package explosions - in a span of 10 days - were connected. Adler says the concern now is that "the methodology has changed".
The two injured men were either biking or pushing bicycles when the explosion occurred.
The latest explosion marks a departure from the first three blasts, which were left at people's homes. Two white men in their 20s were injured in the blast but are expected to be OK.
Manley urged residents to be even more cautious than they have been before.
Austin, with a population of almost 1 million people, is home to the University of Texas and a plethora of tech companies and has been one of the fastest growing major United States cities.
In a tweet Monday, UT Austin police said, "When you get on campus this morning ASK your friends if they've heard about the bombings".
Police reported the number of calls regarding suspicious packages had grown to 849. Classes were resuming Monday after a week off.
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A fourth explosion has rocked Austin, Texas, with two dead and four injured in the blasts.
The device this time entailed "a higher level of sophistication, a higher level of skill", the police chief said.
The three previous bombings happened two-plus weeks in residential neighborhoods east of Interstate 35, which divides the city. The incident happened off of Dawn Song Drive in Travis Country.
He said that it's too soon to say whether Sunday night's bombing that injured two men could have been a response to his call for those behind the bombings to reach out.
The packages on doorsteps weren't mailed: Instead, they were hand-delivered and left on people's doorsteps, likely during overnight hours.
Austin's school district announced that buses wouldn't be going into the Travis Country neighborhood.
A reward of $115,000 has been offered for information leading to the arrest and conviction of the person or persons responsible.
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