Hundreds of agents have joined a hunt for a serial bomber who has targeted random people.

The bombings in March have left two people dead and prompted authorities to launch a massive manhunt.
The bombings in March have left two people dead and prompted authorities to launch a massive manhunt. ( AP )

The hunt for the serial bomber who has been leaving deadly explosives in packages on Austin doorsteps took an even more sinister turn on Monday when investigators said the fourth and latest blast was triggered along a street by a nearly invisible tripwire.

Police and federal agents said that suggests a "higher level of sophistication" than they have seen before, and means the carnage is now random, rather than directed at someone in particular.

"The game went up a little bit – well, it went up a lot yesterday with the tripwire," Christopher Combs, FBI agent in charge of the bureau's San Antonio division, said.

Two people have now been killed and four wounded in bombings over a span of less than three weeks.

The latest blast happened on Sunday night in southwest Austin's quiet Travis Country neighborhood, injuring two men in their 20s who were walking in the dark. 

Authorities have cautioned people not to touch suspected packages which could be sophisticated bombs.
Authorities have cautioned people not to touch suspected packages which could be sophisticated bombs. ( AP )

The three earlier bombings involved parcels that were left on doorsteps and blew up when they were moved or opened.

Those killed were black and one of the injured woman was Hispanic, raising concerns that authorities were slow in reacting because the affected residents weren't white. 

The tripwire twist heightened the fear around Austin, a town famous for its cool, hipster attitude.

"It's creepy," said Erin Mays, 33. "I'm not a scared person, but this feels very next-door-neighbor kind of stuff."

Warnings issued

Authorities repeated prior warnings about not touching unexpected packages and also issued new ones to be wary of any stray object left in public, especially one with wires protruding.

Authorities said they are looking at a range of possible motives, including domestic terrorism or that the bombings were hate crimes.

Local and state police and hundreds of federal agents are investigating, and the reward for information leading to an arrest has climbed to $115,000.

"We are clearly dealing with what we believe to be a serial bomber at this point," Austin Police Chief Brian Manley said, citing similarities among the four bombs. 

While the first three bombings all occurred in a section of town that tends to be more heavily minority, Sunday's was in an affluent area. 

Also, both victims this time are white. 

Echos of Unabomber

The bombings in Austin have led some experts to draw comparison with the 17-year bombing campaign of Ted Kaczynski, more popularly known as the Unabomber.

Ted Kaczynski terrorised American households for years with a similar bombing campaign.
Ted Kaczynski terrorised American households for years with a similar bombing campaign. ( AP Archive )

Kaczynski, who is serving a life sentence, mailed sophisticated explosive devices to his targets. Investigators followed leads for years before finally arresting him in a cabin in secluded woods. 

Danny Coulson, a former FBI assistant director, says the skill used to ensure that the bomb doesn't go off during delivery in Austin is similar to Kaczynski's work. 

Source: TRTWorld and agencies