The trio of the bombs that rocked Austin, Texas over the past several days is believed to be displayed a level of sophistication. Also, they indicate that the bomb-maker or bomb-makers were highly skilled.

Multiple sources briefed on the investigation told 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 were used in three recent bombings. The one was on March 2 and two on March 12. Two people were killed because of the explosions and two others were hospitalized for injuries.

The devices were set up to be detonated by a motion like shaking or jostling, which is why they exploded when they were picked up. The 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.

The motive of the case is being investigated by the authorities, according to the sources.

So far, like source says there were 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. No other theories are being investigated right now.

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.

“There are some specific things that we have seen that make us believe that all three of these packages were made by the same bomb-maker,” Manley said on “GMA.”

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.