The exploding packages that killed two people and injured others after being left at three Austin-area residences contained metal pieces like nails, nuts and bolts, sources close to the investigation told ABC News.

ABC News reports indicated that the devices were set up to be detonated by motions like shaking or jostling, which explains why they may have exploded when picked up by the victims. Sources said the devices had a type of "safe-to-arm" switch, enabling the suspect or suspects to move the devices without blowing themselves up.

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Additionally, ABC News reported that based on the devices' construction and because they successfully detonated without harming the person who left them, investigators believe the bomb maker has a level of expertise.

As of Tuesday, no family connections or other obvious ties have been revealed that could link the victims together, although the Washington Post says family members of two of the victims were friends.

Authorities are also looking at different theories for a motivation for these attacks. One being that it could be racially motivated or drug related.