(BBC) – Police in Austin, Texas, have warned the public that a fourth explosion there may pose a new threat.
They said they were investigating reports the explosion, which injured two people, was activated by tripwire.
Three parcel bombs left on doorsteps have killed two people and injured two others since early March.
The police they say they are working under the belief that the explosions are connected. They warned people in the area to remain indoors.
Austin police chief Brian Manley told reported on Monday that both of those injured in Sunday’s attack were white men in their 20s, unlike the victims in the previous three attacks who were black or Hispanic.
The latest blast came hours after officials announced a new $100,000 (£71,000) reward for information.
The cash reward is on top of an existing $15,000 being offered by the state’s governor.
The two men killed earlier this month were African Americans and police have not ruled out racism as a possible motive.
Chief Manley gave an early morning press briefing to warn of the possibility of a different style of attack.
He said: “It is very possible this device was activated by someone either handling, kicking or coming into contact with a tripwire.
“That changes things in that our safety message has been about the handling of packages. We need the community to have an extra level of vigilance and pay attention to any suspicious device. Do not approach it. Call 911.”
He added: “We do believe that this was a bomb that exploded, but it was not similar to previous explosions. We are working under the belief they are connected but we have not processed the scene.”
Work at the scene will take place after dawn.
The two people hurt in the blast have been treated for non-life threatening injuries.
Mr Manley earlier said he believed the attacks were “meant to send a message”.
He said authorities could not confirm whether a specific ideology was behind the attacks, but appealed to those behind the bombings to contact authorities directly.
Hundreds of federal agents are helping local police, but there has been no arrest. Authorities say they have responded to 735 reports of suspicious items since last Monday.
The first bomb exploded on 2 March, killing Anthony Stephan House, 29, at his home. Two more bombs exploded 10 days later, and were linked to the initial blast.
Draylen William Mason, 17, was killed when he brought a package inside his home. The explosion also critically injured his mother.
Hours later a 75-year-old Hispanic woman was injured by another package. Local media reports this may have been intended for someone else.
The local National Association for the Advancement of Colored People (NAACP) president Nelson Linder told NBC news that the two murdered victims had links to local prominent African-American families and were connected by the same Methodist church.
Investigators say the devices were being left on doorsteps overnight, not being delivered by official methods.
With the city on high alert, a concert by hip-hop band the Roots was cancelled on Saturday after organisers received a bomb threat via e-mail.
They were due to perform at the city’s South by Southwest festival, known as SXSW.
A 26-year-old was later arrested, but police have ruled out any link to the package bombs.