Now, investigators are trying to determine if the person responsible is linked to the trio of bombings this month that killed two people and wounded two others.
"This is a family neighbourhood", Joe said. "Unity, vigilance & objective is the order of the day".
The earlier blasts killed two people and injured two others and happened in other parts of the city.
"We are clearly dealing with a serial bomber", Austin Police Chief Brian Manley said in a Monday press conference. At this point, information is preliminary, he said early Monday morning, and police have yet to fully process the scene.
Fred Milanowski, agent in charge of the Houston division of ATF, said the latest explosive device was anchored to a metal yard sign near the head of a hiking trail.
The men, 22 and 23 years old, were taken to a hospital with what police described as "substantial" but not life threatening injuries.
Responding to reports that the latest explosion may have been detonated by a tripwire, Manley said it was possible that it was "activated by someone either handling, kicking or coming into contact with a tripwire that activated the device". The first three bombs were detonated when victims tried to pick up the package. But he says Sunday's differed in that it involved a tripwire whereas the others were package bombs left on people's doorsteps.
The latest device, which used a tripwire, showed a "different level of skill" to the three previous ones, according to the city's police chief.
Early Sunday, police announced they were offering an additional $50,000 for more information leading to the arrest of a suspect or suspects.
All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores. Two white men in their 20s were injured in the blast but are expected to be OK.
Victims in the previous explosions were black and Hispanic, leading police to explore the possibility of a hate crime.
Linder and the NAACP have been working with the police and community members since the second and third bombings went off March 12. Manley said they are now in stable condition.
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The explosive device was on the side of the road.
Two men were wounded in the explosion Sunday night as they walked near a residential street.
Hours before the latest blast, the chief appealed directly to whoever was responsible for the bombs. Residents should call 911 immediately and not approach anything they find suspicious.
Grote said his son, who lives about 100 yards (91 meters) away from the blast, heard the explosion and raced outside.
'Again, airplanes go by and cars backfire so we didn't think much of it.
"This is a quiet neighborhood".
The attack happened in the southwestern Austin neighborhood of Travis Country. "Any tardies or absences due to this situation will be excused", the district posted on Facebook.
Austin Mayor Steve Adler asked residents to remain vigilant after the string of bombings left the Texas capital on edge.
He said: "These events in Austin have garnered worldwide attention and we assure you that we are listening. The previous four people injured or killed had been people of color".
Police reported the number of calls regarding suspicious packages had grown to 849.
South by Southwest, the huge film, interactive and music festival, wrapped up Sunday, but a bomb threat was made against it Saturday that resulted in the cancellation of a concert featuring The Roots. Download the KVUE app on iOS and Google Play to get those alerts.