CHICAGO: Texas police said Monday that a bombing which left two people injured overnight in the state capital Austin was connected to three previous blasts in the city and the "serial bomber" used a tripwire in the latest attack, showing a "higher level of skill."
Police also said they have been unable to determine a motive for the string of bombings which have killed two people in Austin and put the city of nearly one million on edge.
"As we said from the very beginning, we were not willing to classify this as terrorism, as hate, because we just don't know enough," police chief Brian Manley told reporters.
In the earlier bombings, two African-American men were killed by packages left on their doorsteps, raising the possibility of a hate crime. A 75-year-old Hispanic woman was also injured in a blast.
But Manley said the latest bombing seriously wounded two white men in their 20s as they walked along a sidewalk in southwest Austin. He said it appeared "random" and was triggered by a tripwire.
"We're clearly dealing with what we expect to be a serial bomber at this point based on the similarities between now what is the fourth device," the police chief said.
"(But) what we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night an attack that would have hit a random victim that happened to walk by," he said.
"So we've definitely seen a change in the method that this suspect or suspects are using," the police chief said.
Police believe they are "now dealing with someone who's using tripwires, shows a higher level of sophistication, a higher level of skill," he said.
Manley appealed to the bomber to contact the authorities and to the public to report anything suspicious.
"People need be vigilant, pay attention," he said. "Pay attention to your surroundings."
'We're going to stop it'
Police said they were increasing the reward offered for information leading to an arrest, bringing the total city and state bounty money to $115,000.
"We need every tip, every piece of information, however inconsequential you may think it is," Manley said.
Police cordoned off a half-mile (0.8 kilometer) radius of a residential neighborhood of southwest Austin after the latest explosion.
Area residents were told to stay indoors until police determined it was safe.
An exploding package killed a 39-year-old African-American man on March 2. A 17-year-old African-American man was killed on March 12 and the Hispanic woman was injured the same day.
All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores.
A task force of hundreds of agents are working the case, including criminal profilers and experts from the Federal Bureau of Investigation (FBI) and the Bureau of Alcohol, Tobacco and Firearms (ATF).
Austin Mayor Steve Adler said the manhunt for the bomber was the "highest priority."
"We have some of the best law enforcement folks around dealing with this," Adler said on ABC's "Good Morning America." "There's an army of federal agents. We have state resources.
"We're going to find out who is responsible for this and we're going to stop it," he said.
Since the bombings began city police have responded to some 700 suspicious package calls, the Austin-American Statesman newspaper said.