Police in Austin, Texas were searching Tuesday for a 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 and injuring two others. Another package exploded on March 2, killing a 39-year-old man.
Police believe the attacks were related. All of the cardboard packages were hand-delivered, not sent through the mail.
Investigators have so far not announced any significant progress in identifying a suspect. A reward for information leading to an arrest was raised from $15,000 to $65,000.
"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," the police chief, 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," Manley said.
A task force of some 100 personnel from multiple agencies has been assembled for the case, including criminal profilers and evidence experts from the Federal Bureau of Investigation (FBI) and explosives specialists from the Bureau of Alcohol Tobacco and Firearms (ATF).
"We will be here... until we resolve this and make sure there are no other explosive devices in the city," FBI special agent Christopher Combs told a late afternoon news conference.
The ATF said it is looking for pieces of the explosive devices at the crime scenes and will attempt to reconstruct the packages in a laboratory in California.
As the investigation continued, police received some 265 calls from worried residents reporting suspicious packages. All were false alarms, according to Manley.
"We're not going to leave any stone unturned, and we're going to follow up every lead," the chief said.
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 were 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 African-American church in Austin, while the other was the grandson of a close friend of the same pastor.
"Somebody knew the connection," the retired pastor, Freddie Dixon, told The Post.
"It's not just coincidental."
The Austin police chief acknowledged the possible link and said investigators were following up.
"One of the key things we're trying to understand is is there a connection between all of these victims," Manley said.
"We're just not going to ignore the fact that the three victims that were targeted specifically, that we know of, were people of color," he added.
"That does not indicate that it was a hate crime, but we're not going to rule that out."