Investigators believe that the trio of bombs that rocked Austin, Texas over the past several days displayed a level of sophistication, indicating that the bomb-maker or bomb-makers were highly skilled.
Multiple sources briefed on the investigation told ABC News that the explosive devices were constructed with “enhancers,” like nails, nuts, bolts and other metal pieces that were packed inside to generate shrapnel.
The bombs in question were used in three recent bombings -- one on March 2 and two on March 12. Two people were killed as a result of the explosions and two others were hospitalized for injuries.
The devices were set up to be detonated by motion like shaking or jostling, which is why they exploded when they were picked up. They devices also had some sort of safety switch, which enabled the bomber to move the devices without blowing themselves up, the sources briefed on the investigation told ABC News.
All three of the bombs involved in the recent incidents did explode, though one of them exploded after being carried inside. In that instance, the bomb did not explode instantly after being picked up.
Authorities are trying to determine the motive in the case and are so far at a loss, according to the sources.
So far, the sources said that no family connections or obvious other ties have been found to connect the victims to each other.
One theory that's being explored is that the explosions were racially motivated. Another is that they may be drug-related, the sources said.
This morning, Austin police chief Brian Manley told "Good Morning America" that the focus has been on "post-blast analysis" and they have not been able to identify a suspect yet.
Officials are urging residents to call 911 if they see a suspicious package or receive an unexpected delivery to their home. This morning, Manley said that Austin police had received 150 such calls.
ABC News' Meghan Keneally contributed to this report.