After fourth bombing, Austin police believe they're dealing with a "serial bomber"

Theory Emerging on Motive of Austin Bombings

Police Plea to Austin Bomber: 'We Want to Listen to You'

Police in Texas fear a serial bomber planted four powerful explosive devices that have killed two people and injured four others this month, raising fears in the state capital Austin of another attack.

Cops told residents to stay at home until 10am local time today while the Austin Independent School District said it would not be able to get school buses into the Travis County neighbourhood due to the increased police presence.

Cops say they believe the device is linked to three previous package bombs that killed a man, teenage boy and injured an elderly lady in Austin.

The three prior bombs were left as packages on doorsteps, while Manley said Sunday's device was different: It was placed on the side of the road.

An FBI spokesperson said at the briefing that 350 special agents were now in Austin to help with the investigation.

An Austin police officer directs a vehicle away from the scene of an explosion in Austin, Texas, Sunday, March 18, 2018.

"That is one reason why the concerns about it permeate across the entire country, but it is also a reason why the entire country is galvanizing resources right here in Austin to make sure we solve this crime as quickly as possible", he said. Grote said one of them was riding a bike in the street and the other was on a sidewalk when they crossed a tripwire that he said knocked "them both off their feet".

The device this time entailed "a higher level of sophistication, a higher level of skill", the police chief said.

He said it appeared "random" and was triggered by a tripwire.

Authorities have seemed at a loss to explain who could be setting off the devices or why, saying only that the bombs were sophisticated and that the attacks could have been motivated by racial bias, although they acknowledged that this is only a theory.

The explosions happened far from the main South By Southwest festivities, though a separate bomb threat Saturday led Bud Light to cancel a downtown concert by hip-hop band The Roots.

Sunday's bomb injured two men - aged 22 and 23 - who were walking through a residential area in the Travis Country neighborhood.

'Again, airplanes go by and cars backfire so we didn't think much of it.

In the GMA interview, Manley said more than 500 investigators, including 350 Federal Bureau of Investigation agents, continue to chase down every lead, but "at this point we don't have specifics leading us to one suspect".

"What we have seen here is a significant change from what appeared to be three targeted attacks to an attack that could have harmed anyone", Manley said.

Police said all four incidents have similarities but have not yet made a broader connection or identified a motive.

"We are working under the belief that this is related to the other bombing incidents that have occurred in our community over the last couple weeks", said Manley, according to The Washington Post.

"We said from the beginning we weren't going to rule anything out because when you rule something out, you limit your focus", Manley said on Good Morning America March 19.

Those three blasts all happened after someone left explosives-laden packages on the victims' doorsteps.

The coalition organized a town hall last week to bring together community members and law enforcement after the first three bombings.

He then said he wanted to tap into the psyche of the bomber or bombers.

Hours after the explosion that killed Mason, 75-year-old Esperanza Herrera also received a exploding package bomb. At this point we don't have any specifics leading us to one suspect.

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