Austin police say they are continuing their search for a suspect after a fourth package bomb in the city showed a “different level of skill” than the previous three attacks this month.

Police Chief Brian Manley is urging residents in a south-west neighbourhood of the city to be cautious of suspicious packages after the most recent blast, which injured two cyclists on Sunday night when they triggered a trip wire. Police say the four attacks are probably connected.

“The belief that we are now dealing with is someone who uses trip wires shows a higher level of sophistication, a higher level of skill,” Mr Manley said during a Monday morning press conference.

Authorities described an evolving strategy by the bomber, who they have so far been unable to identify despite numerous tip offs.

They noted that the bombs may now be triggered by twine, fishing line, or any number of trip wires, and that people should be cautious to ensure they do not accidentally hit one of those lines. Parents were warned that children could be at risk, and that extra caution should be taken to ensure young kids do not wander off where a bomb could have been installed.

“We are clearly dealing with what we expect to be a serial bomber at this point,” Mr Manley said, indicating that the fourth device shared similarities with the previous three blasts.

Fourth Austin bombing: in pictures Show all 8 1 /8 Fourth Austin bombing: in pictures Fourth Austin bombing: in pictures Investigators work at the scene of a bombing on Dawn Song Drive in the Travis Country neighborhood. Rex Fourth Austin bombing: in pictures Police tape marks off the neighborhood where the package bomb went off. It is the fourth similar bombing in three weeks in the Austin area. Getty Fourth Austin bombing: in pictures A police crime scene van arrives near the site of the explosion. Police warned nearby residents to remain indoors overnight as investigators looked for possible links to other deadly package bombings elsewhere in the city this month. AP Fourth Austin bombing: in pictures FBI and police investigate a bombing at the intersection of Republic of Texas and Mission Oaks boulevards. Rex Fourth Austin bombing: in pictures Officials work on the site of the explosion. AP Fourth Austin bombing: in pictures Members of Austin Police Department block off roads, Reuters Fourth Austin bombing: in pictures Police lines are seen blocking off part of Republic of Texas Boulevard. Reuters Fourth Austin bombing: in pictures Police maintain a cordon near the site of the explosion in southwest Austin. Reuters

Mr Manley had previously said that school buses would not be sent into the neighbourhood on Monday morning, and asked that his investigators be given space to process the scene of the crime in daylight.

“We will not be able to send school buses into the neighbourhood on Monday,” he said on Sunday night after the fourth explosion. “In addition to that, we’re going to ask the residents in the Travis County neighbourhood to stay in your homes tomorrow morning and give us the opportunity to process the scene once the sun comes up.”

Austin Mayor Steve Adler said that he thinks the latest explosion will raise anxieties in his city, which he said is justifiable even though he thinks residents should be reassured by the massive response from police and federal agencies.

"That concern is legitimate and real," said Mr Adler. "That anxiousness is going to continue until we can find the answer."

The most recent victims of the blast were reportedly two individuals in their 20s, and both were white men, police have said. That is compared to the previous blasts, which appeared to target minorities in the community and left two black men dead, and left some speculating that the attacks were race related.

When asked if these attacks are terrorism, or race related, Mr Manley said that investigators have not yet identified a motive.

The Sunday night explosion differs from the previous three blasts, because the previous bombs were dropped off on doorsteps. The previous victims handled the packages as if they had received them in the mail – the United States Postal Service and other services have said they did not process the packages in their systems – and the Sunday night blast followed after someone came into contact somehow with a trip wire.

A $115,000 (£82,000) reward has been offered for information leading to the arrest or conviction of people responsible for the blasts.

FBI special agent Christopher Combs, who addressed the media alongside Mr Manley, said that his bureau had sent 300 agents to Austin to help investigate the attacks in what he said was an “unprecedented” response in the city.

The first package bomb detonated on 2 March after 39-year-old Anthony Stephan House picked up the bomb from his front porch. Mr House was transported to a nearby hospital where he was pronounced dead. Police at the time determined that the incident was isolated, but noted that the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) were assisting the department with an investigation into the matter.

Two more bombs were detonated on 12 March, leading Austin police to say that they believe the three incidents were connected. The first explosion that day killed a 17-year-old resident, who found a package on his front step that morning before taking it into his kitchen where it detonated. A woman in her 40s was also seriously injured by that explosion. Hours later, as investigators were still combing through the crime scene, a third bomb was detonated, sending a 75-year-old Hispanic woman to the hospital in a “critical, but stable condition”.