Police fear four explosions this month around Austin, Texas, are the work of a serial bomber.
Investigators say they have no clear idea what motivated the series of attacks, which began 17 days ago, and have publicly reached out to the bomber, urging him or her to contact them and explain his demands.
The latest bomb, which injured two people on Monday (Sunday US time), may have been activated by a trip wire, a more advanced design than the three previous explosions that were set off when victims handled packages left on doorsteps.
Those packages killed two earlier this month.
“We are clearly dealing with what we expect to be a serial bomber at this point,” Austin police chief Brian Manley said.
“We have seen similarities in the devices that exploded here last night and the other three devices.”
The latest device triggered by a tripwire shows “a higher level of sophistication, a higher level of skill,” he said.
The men, one aged 22 and the other 23, suffered non-life threatening injuries after they came upon a suspicious device on the side of a road in a residential neighbourhood, officials said.
The three earlier parcel bombs that exploded in three separate east side neighbourhoods killed two African-American males and left a 75-year-old Hispanic woman fighting for her life.
Police have received more than 700 calls about suspicious packages since the three parcel bombs, but authorities have not found any that posed a security risk, Mr Manley said.
More than 500 federal agents are reportedly involved in the investigation, including from the FBI and the US Bureau of Alcohol, Tobacco, Firearms and Explosives.
“There is an army of law enforcement folks on the scene right now,” Austin Mayor Steve Adler told CNN.
“I am confident that we’re going to find whoever is responsible for this and then we’re going to stop it.”
FBI agents have swept the relatively affluent neighbourhood called Travis Country where Sunday’s bomb exploded and asked residents for home surveillance videos, residents said.
Comparisons to Unabomber
The attacks have been compared to the infamous ‘Unabomber’, Ted Kaczynski, who was convicted over a US-wide bombing spree that spanned nearly two decades, ending in the mid-1990s.
The key similarity is that Kaczynski also used parcel bombs. He was found to have killed three and injured several others.
Kaczynski was labelled a domestic terrorist, as his purported aim – detailed at length in a manifesto sent to The Washington Post and The New York Times – was to protest against the rise of modern technology.
Kaczynski is serving eight life sentences in a maximum security prison in Colorado.
A child prodigy born in Chicago, he was accepted into Harvard, graduated in 1962 and went on to earn a PhD in mathematics from the University of Michigan.
After working as an assistant professor of mathematics at the University of California Berkeley, Kaczynksi dropped out and moved to a remote cabin in Montana.
His bombing campaign, which started in 1978, triggered one of the largest manhunts in US history.