Chicago (AFP) - Police in Austin, Texas were searching Tuesday for a skilled bomb maker as the US city remained on edge following a series of explosive packages sent to homes, killing two people.
Two packages left at residents' doorsteps detonated Monday, killing a 17-year-old boy and injuring two others. Another package exploded March 2, killing a 39-year-old man.
Police believe the attacks were related. All the packages were hand-delivered, not sent through the mail.
Investigators gathered forensic evidence at the two latest crime scenes, but have not made any progress on identifying a suspect, Austin's police chief said Tuesday.
"What we do know is that the individual or individuals that are involved in this, these suspects, they do have a certain level of skill," Brian Manley told Fox News.
"To be able to construct a device like this and then deliver that device to your target, without having it explode either during construction or during delivery, does take a certain level of sophistication," he said.
As the investigation continued, police received some 150 calls reporting suspicious packages. All were false alarms, according to Manley.
The attacks came as the world's attention was focused this week on Austin's annual South by Southwest (SXSW) gathering -- a series of festivals featuring celebrities from the media and entertainment worlds.
Festival organizers said their security staff was on high alert.
"The substantial security operation already in place for SXSW has been instructed to be extra vigilant," a festival spokesperson said in a statement.
Meanwhile, Austin's African American community was pondering a potentially troubling connection in the first two attacks.
The Washington Post newspaper reported that one of the people killed was the stepson of a former pastor at a historic black church in Austin, while the other was the grandson of a close friend of the same pastor.
"Somebody knew the connection," the pastor, Freddie Dixon, told The Post. "It's not just coincidental."
Police suggested early Monday that the attacks could be hate crimes, but later emphasized that they were considering all potential motives.
"It's too early for us to know," Austin Mayor Steve Adler told ABC's Good Morning America program on Tuesday.