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 a bomb which detonated 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?” he asked.

Manley said investigators will “have to determine if we see a specific ideology behind this”.

He added that 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.

Manley warned people not to touch suspicious bags, boxes or backpacks, especially if they have wires protruding.

He said: “We want to put out the message that we’ve been putting out and that is, not only do not touch any packages or anything that looks like a package, do not even go near it at this time.”

Manley also said authorities had worked to “clear” a suspicious backpack found in the area that was part of a separate report.

Mayor Steve Adler said the latest explosion would only raise anxieties in the city.

“That concern is legitimate and real,” Adler said, adding that residents should be reassured by the massive police response to the attacks.

More than 500 officers, including agents from the FBI and other federal agencies, have conducted 236 interviews in following up on 435 leads.

They blocked entrances to the neighbourhood near to the latest blast site and put up yellow tape about half a mile from the home where it happened.

Despite warnings to stay indoors, nearby neighbours milled around just outside the cordon.

Austin’s school district announced that buses would not be going into the Travis Country area because of police activity and that any absences “due to this situation” will be excused.

After sunrise, officers scoured the area for anything suspicious.

The latest explosion came hours after authorities raised the reward by $50,000 (£35,000) for information leading to the arrest of whoever is responsible for the first three explosions.

The total now stands at $115,000 (£82,400).

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.

TWO people have been injured after Texas’ capital city Austin was rocked by its fourth bombing this month.

The latest device, which used a tripwire, showed a “different level of skill” to the three previous ones, according to the city’s police chief.

Brian Manley told a US national television station that both men who were injured in Sunday night’s explosion in the south-western Austin neighbourhood of Travis Country are white, unlike the victims in the three earlier attacks, who were black or Hispanic.

Both of the men injured are in their 20s and were transported to a local hospital with what were described as non-life-threatening injuries.

The first of the four explosions to hit Austin this month was caused by a package bomb that detonated at a house in the city’s north-east on March 2, killing a 39-year-old man.

Two more package bombs then exploded farther south on March 12, killing a 17-year-old, wounding his mother and injuring a 75-year-old woman.

Police said all three of those explosions were likely to be related and involved packages that had not been mailed or delivered by private carrier but left overnight on doorsteps.

Manley had said the earlier three bombings are related and could be crimes of hatred.

But investigators now maintain that they are not ruling out any possible motive.

Police and FBI officers will consider any motive or any clear idea of “what the ideology is behind this”.

The latest explosion came hours after authorities raised the reward by $50,000 (£35,000) for information leading to the arrest of whoever is responsible for the first three explosions. It now totals $115,000 (£82,400).

Residents were warned to remain indoors and to call police if they needed to leave their homes before 10am.

After sunrise, officers scoured the area for anything suspicious.

Travis Country is far from the sites of the earlier bombings, which occurred over two weeks in residential areas east of the Interstate 35 road, which divides the city.

At a news conference hours after Sunday’s blast, Manley repeated his warning for people to not pick up or approach suspicious packages.

He said: “We want to put out the message that we’ve been putting out and that is, not only do not touch any packages or anything that looks like a package, do not even go near it at this time.”

Manley also said authorities had worked to “clear” a suspicious backpack found in the area that was part of a separate report.

Police kept residential streets on lockdown, gradually expanding their barricades and closing off all roads into the area.

Before daybreak, Austin police sent another alert to mobile phones, advising residents to continue staying indoors and to call police if they needed to leave their homes before 10am.

Austin’s school district announced that buses would not be going into the Travis Country area because of police activity and that any absences “due to this situation” will be excused.

More than 500 officers, including agents from the FBI and other federal agencies, have conducted 236 interviews in following up on 435 leads.

They blocked entrances to the neighbourhood near to the latest blast site and put up yellow tape about half a mile from the home where it happened.

Despite warnings to stay indoors, nearby neighbours milled around just outside the cordon.

Although the explosion was severe enough to injure two men, people who lived in the vicinity of the blast said they had not seen or heard much.