Police in Texas fear a serial bomber planted four powerful explosive devices that have killed two people and injured four others this month, raising fears in the state capital Austin of another attack.
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 what his demands are.
The first three devices were parcel bombs dropped off at night in front of homes on the city's east side, with the fourth a trip wire device that went off in a west side neighbourhood on Sunday. All four devices were similar, police say.
"We are clearly dealing with what we expect to be a serial bomber at this point," Austin Police Chief Brian Manley told a news conference near the site of Sunday's bombing.
Two men were injured on Sunday by the latest bomb, which police said may have been activated by a trip wire across a footpath. It was a more advanced design than the previous bombs.
The men, 22 and 23 years old, were taken to a hospital with what police described as "substantial" but not life-threatening injuries.
Manley said more than 500 federal agents were involved in the investigation, including from the Federal Bureau of Investigation and 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 said on 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.
