A fourth explosion in less than a month has hit Austin, Texas, with police hunting for what they believe to be a "serial bomber" or bombers with all of the incidents believed to be linked.

Police said that the cause of the latest explosion is believed to have been triggered by a tripwire in the Travis County neighbourhood of the city. Police have appealed for video surveillance footage from residents and called on them to stay clear of any suspicious packages. They have also called on all those in the areas to stay in their homes so the scene of the Sunday night blast can be fully investigated.

The two victims of the latest explosion are two white males, in their 20s who are being treated for non-life threatening injuries.

Austin Police Chief Brian Manley asked the city “to have an extra level of vigilance and pay attention to any suspicious device whether it be a package, a bag, a backpack, anything that looks out of place and do not approach it”.

How many bombings have there been and where?

Four explosions have taken place in March alone, all using packages not delivered by the US Postal Service nor any other US mail carrier service, police said.

The first was on 2 March when a package was delivered to the northwest Austin home of Stephan House, 39, who later died from his injuries in hospital.

On 12 March another package bomb was sent to the home of Draylen Mason, aged just 17, killing him. The package exploded in the kitchen of his east Austin house just 12 miles from the previous explosion.

That same day, just five hours after the bomb killed Draylen, another package bomb exploded at the home of a 75-year-old Hispanic woman called Esperanze Morena Herrera in the southeastern part of the city. She was treated for her injuries.

On 18 March, the most recent package bomb exploded in the western part of the city, injuring two. The package exploded in the street itself, not inside a home like the others.

The latest blast demonstrated what police said was a “higher level of sophistication” than the package bombs used in the previous three attacks.

Who are the victims?

Mr House about an hour after the 2 March blast that occurred on the front porch. Police had been cautious in ruling it a homicide at the time, unsure if Mr House himself had constructed the explosive device inside the package.

Mr House and Draylen were members of a close-knit African-American church community and their families reportedly knew each other, NBC News reported.

Both were parishioners at the Wesley United Methodist Church, an organisation founded over 150 years ago by freed slaves.

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

After the first two bombings police were looking into the theory that the deaths might be race-related, or a possible hate crime.A local representative of the National Association for the Advancement of Colored People (NAACP) Nelson Linder suggested to NBC News that Ms Herrera, who remains in hospital in critical condition, was not the intended target of the third bombing.

“The intended target was another person who might be connected to the House and Mason families,” Mr Linder said, but would not elaborate or reveal that person’s identity. Police have not confirmed this.

However, the victims of the fourth bombings are two white men, aged 22 and 23. The Austin police chief Mr Manley said that investigators have yet to establish a motive.

“Is this terrorism? Is this hate-related?” Mr Manley said. He said investigators will “have to determine if we see a specific ideology behind this”.

Are there any suspects?

No suspects have been named as yet, with police calling for help from residents with video footage and asking for them to continue to phone in tips - particularly in regards to the latest incident - as they look to build a picture of what happened

Mr Manley said the police had followed up with a number of persons of interest following tips, but so far there are no concrete suspects.

The Austin Police had announced a $100,000 reward for information leading to the arrest of anyone connected with the bombings, on top of the Texas Governor's $15,000 reward which was announced after the second bombing.

What is happening now?

Mr Manley said the latest bombing differed from the previous three. “We're not believing that this was similar to previous ones, as in packages left on doorsteps. But instead, this was some type of suspicious package that was left on the side of the road, that detonated and injured these two men," he 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.

FBI investigators were on the scene Monday morning but have not commented on the matter. The bombings have taken place during of the South by Southwest Festival (SXSW), but police said they have nothing to do with the multi-day, multi-venue music, film, and social cause event.

Austin 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.