The explosive device yesterday adds to the uncertainty in Austin, which has been on edge since previous bombings killed two people and injured two others, one seriously.
Authorities have seemed at a loss to explain who could be setting off the devices or why, saying only that the bombs were sophisticated and that the attacks could have been motivated by racial bias, although they acknowledged that this is only a theory.
The latest explosion injured two white men - one 22, the other 23 - walking through part of Austin's southwest area, far from where the first three devices detonated.
The explosive device was on the side of the road. The previous packages were all left at people's homes, authorities said.
"With this tripwire, this changes things," said Christopher Combs, special agent in charge of the FBI's San Antonio office. "It's more sophisticated. It's not targeted to individuals. We're very concerned that with tripwires, a child could be walking down the sidewalk and hit something."
Authorities have previously described the explosives as the sophisticated work of someone who knows what they are doing, saying the bombers have been able to assemble and deliver the packages without setting them off. After telling residents to remain wary of unexpected or suspicious packages, authorities were now urging broader caution.
"We're even more concerned now that if people see something suspicious, they stay away from it altogether and contact law enforcement," said Fred Milanowski, special agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. "Because if they move that package or if they step on that tripwire, it's likely to detonate."
Milanowski said devices using tripwires are activated when any pressure is applied to the wires, and he said that can include people "tripping over it or picking up the package".
The two men wounded yesterday were taken to a hospital with serious but non-life-threatening injuries, and Manley said they were in stable condition today. Residents described the neighbourhood as a wealthy area filled with families.
"It's shocking," said Austin City Councillor Ellen Troxclair, who represents the district. "The trip wire definitely instilled some fear into this neighbourhood. They just want to know what's going on."
Many in the southwest Austin neighbourhood previously felt that they were immune from the terror that had shaken other parts of town.
"It appears that no one is safe, and I'm very fearful for our community," said Richard Herrington, 75, who was watching the NCAA men's basketball tournament when he heard the explosion. "It's very concerning that this person is becoming more sophisticated."
The first two bombs killed black people - a 39-year-old construction worker and a 17-year-old high school student - related to prominent members of Austin's African-American community who were also close friends. The third bomb seriously injured a 75-year-old Hispanic woman, but it was addressed to a different home and apparently exploded when she was carrying it, said two people familiar with the case.
The first three explosions detonated in the eastern part of Austin, where black and Hispanic residents live. Some in the area questioned whether the initial blast would have prompted more urgency had it gone off in a more affluent, predominantly white neighbourhood.
Police said they are still considering whether some of the bombings were hate crimes.
"We've said from the beginning that we're not willing to rule anything out, just because when you rule something out you limit your focus," Manley told ABC. "This does change the concerns that we had initially, although we have still not yet ruled it out until we understand what the ideology or motive is behind the suspect or suspects."
Manley said that police do not have evidence leading them to a particular suspect, and he reiterated his plea to the public for tips and information.
