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Fear has escalated across the US city of Austin after the fourth bombing this month.
Key points:
- The two injured men have been hospitalised with non life-threatening injuries
- The three previous bombings were parcel bombs left on doorsteps
- Police are offering a $US115,000 reward for information leading to an arrest
The latest blast in the Texas capital was thought to have been triggered by a tripwire, demonstrating what police said was a "higher level of sophistication" than the package bombs used in previous attacks.
Two men, thought to be in their 20s, suffered non-life threatening injuries and were taken to hospital on Sunday (local time) after they came upon a suspicious device on the side of a road, Austin police chief Brian Manley said during a press conference.
He said investigators had yet to establish a motive and they would "have [to] determine if we see a specific ideology behind this."
"We are clearly dealing with what we believe to be a serial bomber at this point," he said.
The possibility that the roadside bomb was triggered when someone handled, kicked or came in contact with a tripwire, differed from the previous explosions that were set off when victims handled packages that were left on doorsteps, Mr Manley said.
"We now need the community to have an extra level of vigilance and pay attention to any suspicious device," Mr Manley said.
"Given that there may have been a different triggering mechanism in this device, we wanted to get that out as early as possible."
The injured men were riding or pushing bicycles when the explosives detonated.
The first of the four explosions was caused by a package bomb that detonated at a north-eastern Austin home on March 2, killing a 39-year-old man.
Two other package bombs exploded further south on March 12. The first killed a 17-year-old and wounded his mother, and the second injured a 75-year-old woman.
'Concern is legitimate and real'
Authorities were canvassing the area in search of anything suspicious, and residents were warned to remain indoors and call emergency services if they needed to leave their homes.
Mayor Steve Adler said the latest explosion only further raised anxieties in the city.
"That concern is legitimate and real," Mr Adler said, adding that residents should also be reassured by the massive police response to the attacks.
Droves of federal agents are investigating, along with Austin police.
"That anxiousness is going to continue until we can find the answer," Mr Adler said.
Spring break ends Monday for the University of Texas and many area school districts, meaning people who were out of town, have returned home to heightened fears.
The university's campus police warned returning students to be wary, saying, "we must look out for one another."
None of the four attacks happened close to the university's sprawling campus near the heart of Austin.
Police kept residential streets near Sunday night's blast on lockdown, gradually expanding their barricades.
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.
A reward of $US115,000 ($149,000) has been offered for information leading to the arrest and conviction of the person or people responsible.
AP/Reuters
Topics: law-crime-and-justice, police, united-states
First posted