A suspected serial bomber has struck again in Austin, Texas, after the city was hit by its fourth explosion this month.
The new blast, which left two people in hospital, was triggered by a tripwire and demonstrated what police said was a "higher level of sophistication" than the package bombs used in the previous three attacks.
Two men in their 20s are in a stable condition after they were injured by an explosion as they walked along a street on Sunday night. Three earlier bombings, starting on March 2, have left two people dead and two others injured.
Austin police chief Brian Manley said: "We are clearly dealing with what we believe to be a serial bomber at this point, based on the similarities between now what is the fourth device" and the previous ones.
He said investigators have yet to establish a motive.
"Is this terrorism? Is this hate-related?" Mr Manley asked.
He said investigators will "have to determine if we see a specific ideology behind this".
Mr Manley said the blast involved a tripwire, unlike the first three attacks, in which package bombs were left on people's doorsteps. That represents a "significant change", in that the previous bombings appeared targeted, while the latest one would have hurt any random person walking by.
The device this time entailed "a higher level of sophistication, a higher level of skill", the police chief said.
Sunday night's explosion happened in the south-western Austin area of Travis Country. That is far from the sites of the earlier bombings, which occurred over two weeks in residential neighbourhoods east of the Interstate 35 road.
The two injured men, who were aged 22 and 23, are white, unlike the victims in the earlier blasts, who were black or Hispanic.
Mr Manley warned people not to touch suspicious bags, boxes or backpacks, especially if they have wires protruding.
Mayor Steve Adler said the latest explosion would only raise anxieties in the city.
"That concern is legitimate and real," Mr Adler said, adding that residents should be reassured by the massive police response to the attacks. Hundreds of federal agents are also investigating, along with Austin police.
"That anxiousness is going to continue until we can find the answer," Mr Adler said.
The latest explosion came hours after authorities raised a reward for information leading to the arrest of whoever is responsible for the first three explosions by 50,000 dollars (€40,500), to 115,000 dollars (€93,200).
The PGA's Dell Technologies Match Play golf tournament is scheduled to begin in Austin on Wednesday, and dozens of the world's top players are set to arrive the day before.