What investigators are looking for to crack the Austin 'serial bomber' case

The desperate search for the suspect or suspects who have put Austin residents on edge with a series of explosives is now likely focused on the few clues police have on hand.

Fred Milanowski, a special agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives who is on the ground in Austin, told ABC News that officials are looking at “a couple of people of interest” but added they are still actively calling for the public’s help.

The most obvious avenue for investigation at this point is examining the remnants of the four devices that have exploded in Austin over a 17-day span. The explosions have killed two people and injured four others.

“Every bomber leaves a signature because they tend to make their devices the same way every time 'cause once they’ve perfected one they don’t want to take a chance of accidentally detonating something,” Milanowski told ABC News.