Austin police have received more than 1,200 suspicious package calls since bombings started

Show Caption Hide Caption Raw: Explosion in Fedex plant near San Antonio The San Antonio Texas Fire Department says a package bomb has exploded at a FedEx distribution center in Schertz, Texas, hurting 1 person, a FedEx employee who apparently suffered a non-life-threatening "percussion-type" injury from the blast. (March 20)

A series of explosions across Austin, Texas, have left residents on edge while law enforcement conducts an extensive search to find the bomber. The Austin Police Department received 1,257 suspicious package calls since the first of four bombings occurred March 12. About a third of those — 420— occurred from 8 a.m. Monday to 8 a.m. Tuesday, after a tripwire-triggered explosion injured two bicyclists Sunday evening.

Austin Police Chief Brian Manley said the bombings, which have killed two and injured four, were designed to "send a message" and urged the person or people behind the bombings to step forward.

APD responded to 420 suspicious package calls between 8 a.m. yesterday and 8 a.m. today bringing the total number to 1,257 calls since approximately 8 a.m. on March 12. — Austin Police Dept (@Austin_Police) March 20, 2018

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Roughly 500 local, state and federal law enforcement officers are working to track down the "serial bomber," Manley said. All the while, officials have urged residents to report anything suspicious. U.S. Department of Homeland Security Secretary Kirstjen Nielsen told Austin residents to be "very cautious, obviously," and to follow the guidance of local authorities.

Early Tuesday, a package from Austin exploded at a FedEx facility near San Antonio, injuring a worker. FBI San Antonio spokeswoman Michelle Lee said it would be "silly" for the agency not to believe it was connected to the Austin explosions. FedEx said the "individual responsible" also sent another package bomb.

APD is asking the public to remain vigilant and report anything suspicious. If you come across ANYTHING that looks suspicious, DO NOT touch, handle or disturb it. Keep a safe distance and call 9-1-1 immediately. pic.twitter.com/Dk0uWVcJiP — Austin Police Dept (@Austin_Police) March 20, 2018

Ex-FBI agents say law enforcement officers are likely sifting through microscopic forensic evidence and analyzing behavioral evidence in order to find a suspect or suspects.

Bill Jonkey, a former FBI bomb technician, said law enforcement must have found some commonality in the bombs for it to connect the cases. A timing device or other pieces of a bomb can lead police to consistencies across a number of bombings.

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"You don't go to the store and buy a bomb," said ex-FBI agent Gregory M. Vecchi, an assistant professor of criminal justice at Missouri Western State University. "You have to put it together."

Vecchi said this allows authorities to track bomb part purchases to certain stores, which can lead law enforcement to a credit card and perhaps a person.

Police also are considering a rhyme or reason to the targets. Were they chosen at random or did someone target them?

"These are difficult," Vecchi said. "It's very resource intensive."

A bomb explosion can result in hundreds of pieces of shrapnel, which must be sent to a lab for examination.

Jonkey said it's not unusual for a large number of public reports following cases like Austin. That's a good thing, he said. But it also means law enforcement must investigate each one.

"It's scary," Vecchi said. "You cannot ignore any one of those allegations or leads.

Follow Sean Rossman on Twitter: @SeanRossman

John Moritz and Eliza Collins contributed to this report.