A serial bomber is suspected of planting four bombs detonated this month around Austin, Texas, that have killed two people and injured four others.
The city’s Police Chief Brian Manley told a news conference: "We are clearly dealing with what we expect to be a serial bomber at this point.
"We have seen similarities in the devices that exploded here last night and the other three devices."
Two men were injured last night by the latest bomb, which police said may have been activated by a trip wire across a footpath.
It is a more advanced design than the previous bombs that were set off when victims handled packages left on doorsteps.
The men, aged 22 and 23, were taken to a hospital with what police described as "substantial" but not life threatening injuries.
The first parcel bomb on 2 March killed 39-year-old African-American Anthony Stephan House.
A bomb last Monday morning killed Draylen Mason, a 17-year-old African-American.
A few hours later, a third bomb injured a 75-year-old Hispanic woman, who has not been identified.
Chief Manley has said police were investigating the bombings as possible hate crimes, but cautioned that the theory may not hold up as the most recent attack did not appear to have targeted specific people and both victims were white.
More than 500 federal agents are involved in the investigation, including from the Federal Bureau of Investigation and US Bureau of Alcohol, Tobacco, Firearms and Explosives.
Yesterday agents swept the relatively affluent neighbourhood called Travis Country where the latest bomb exploded and asked residents for home surveillance videos.
"It's scary," 76-year-old Thad Holt, said recalling that he and his wife had walked close to the bomb site about half an hour before the explosion.
He said: "It's one of those things ... that happens elsewhere."
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, Chief Manley said.
A reward of $115,000 has been offered for information leading to the arrest and conviction of the person or persons responsible.
Austin, with a population of nearly one million people, is home to the University of Texas and a plethora of tech companies and has been one of the fastest growing major US cities.
