The latest blast, which injured two men while they were walking along the road in a residential area, plunged the city further into a frightening mystery that forced residents in the vicinity of the bombing to remain locked in their homes as investigators scoured the area for answers.
Austin Police Chief Brian Manley said they are looking into whether the bomb was triggered when someone handled, kicked or came in contact with a trip wire.
An FBI spokesperson said at the briefing that 350 special agents were now in Austin to help with the investigation.
Police said they are still considering whether some of the bombings were hate crimes. "Communicate with us so that we can put this to an end", Manley said.
The device this time entailed "a higher level of sophistication, a higher level of skill", the police chief said.
"The device's success, despite significantly different design, further suggests that the bombmaker behind these attacks is an accomplished one, and has likely to have received some training, perhaps as a military or police explosive ordnance disposal technician", Stratfor said.More news: NCAA Tournament Predictions: Texas A&M vs. North Carolina 3/18/18
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According to the Office of the Governor, the office has provided almost $1 million to the City of Austin to enhance the capabilities of the Austin Police Department's Bomb Squad over the past two years.
The first two bombs killed black people - a 39-year-old construction worker and a 17-year-old high school student - related to prominent members of Austin's African-American community who were also close friends.
Two bicyclists in their 20s were either riding or walking their bikes on the sidewalk just after 8:30 p.m. local time Sunday, said Police Chief Brian Manley. But the victims on Sunday's explosion were two white men, leaving open the question of whether the victims' identities matter to the bombmaker. "Again do not approach items like that, but instead call 911 to report it so we can go out and make sure it's safe". At this point, information is preliminary, he said early Monday morning, and police have yet to fully process the scene.
"First and foremost, Cecilia and I offer our thoughts and prayers to the victims of these atrocious attacks", Abbott said. Hundreds of federal agents are investigating, along with Austin police.
"We're not believing that this was similar to previous ones, as in packages left on doorsteps".
The PGA Tour, which is staging the Dell Match Play Championship in Austin, Texas, this week, says its security advisers are collaborating closely with law enforcement to try to ensure the safety of players and fans.
"That's why we're pushing the message out there that we need every tip, every piece of information however inconsequential you may think it is", Manley said. "This changes the concerns that we had initially". "I am confident that we're going to find whoever is responsible for this and then we're going to stop it". None of the three bombs, which were not delivered through normal mail delivery, exploded until it was handled.
The victims of the earlier bombings include Draylen Mason, 17, an orchestral musician and notable essay victor, and Anthony Stephan House, a 39-year-old father.