Authorities believe the explosive device was triggered by a tripwire
Residents in the Travis Country area are advised to remain inside until 10am
It follows a string of package bombs that killed two in Austin in the past month
Anthony Stephan House, 39, died on Mar 2 and followed by 17-year-old, Draylen Mason on Mar 12 when package bombs were left at their respective homes
The dead men were both African-American raising speculation the bombing was racially motivated
The number of injured survivors increased has increased with the addition of two Caucasian men in their twenties, in the latest explosion, to Hispanic woman Esperanza Herrera, 75, who was injured in the third explosion, on Mar 12
Feds now say the detonations were likely the handiwork of a serial bomber
Police said latest attack showed ‘different level of skill’ than the package bombs
FBI agents converged on the crime scene on Sunday night. An affluent residential neighborhood of single-family homes
An explosion in Austin that injured two men on bicycles forced police to warn nearby residents to remain indoors overnight as investigators looked for links to three other package bombings in the city this month.
The latest blast occurred around 8.30pm on Sunday in a southwestern Austin residential neighborhood known as Travis Country.
The explosion seriously injured two white men in their 20s when they are believed to have triggered a tripwire when they were pushing or riding their bikes down a sidewalk.
Austin Police Chief Brian Manley told Good Morning America that the latest attack showed ‘a different level of skill’ than the package bombs used in the three prior attacks.
Related: Are the Austin package bomb incidents race related? Questions asked as authorities say families of two Austin package bomb victims knew each other – All affected families are minorities
Agents with bomb sniffing dogs, inspect the scene of the fourth bombing in Austin, TX on Sunday. Police have established a wide cordon and urged residents to remain indoors until receiving an all-clear from cops
‘It is very possible that this device was activated by someone either handling, kicking or coming in contact with a tripwire that activated the device,’ Chief Manley said in an early morning press briefing on Monday.
‘We do believe, based on what we have seen, that it was a bomb that exploded.
‘We do not believe that this was, as previously, a package left on doorstep, but that this was some type of suspicious package that was left on the side of the road and that detonated and injured these two men.
‘We are working under the belief that they are connected to the previous bombings.’
Residents in the Travis Country area were ordered to stay in their homes until Monday morning and no school buses were going to drive through the area to pick up children.
Manley said authorities would keep the surrounding area blocked off until further notice because of overnight darkness and the ‘size of the area that we want to go in and check.’
Police kept residential streets on lockdown, gradually expanding their barricades and closing off all roads into the neighborhood.
Anthony Stephan House and Draylen Mason were the first victims’ of the Austin package bomber
Before daybreak on Monday, Austin police pushed another alert to cellphones advising residents to continue staying indoors and to call 911 if they needed to leave their homes before 10am.
Manley also said authorities had worked to ‘clear’ a suspicious backpack found in the area that was part of a separate report.
‘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 said.
Austin police issued an advisory after the fourth explosion on Sunday
Anthony Stephan House, 39, died on March 2 was the first known Austin resident to be killed by the serial bomber when a package bomb was left at his door step
Package bomb attack victim was college bound 17-year-old music prodigy, Draylen Mason, 17, who died on March 12 when a package bomb was delivered to his home
The alleged serial bomber has had a busy month starting with the Friday, March 2 when Anthony Stephan House, 39, was killed when a package blows up at 6.55am at his home in Northeast Austin>
The second explosion occurred on March 12, killing Draylen Mason, and a woman is seriously injured in a package explosion in the kitchen of a home 12 miles south of the first incident. Four hours later that same day, Esperanza Herrera, 75, was severely injured in a package explosion while visiting her mother’s home, five miles from the Mason home.
In Sunday’s incident, two men in their twenties were seriously injured by an explosion at a home on the 4800 block of Dawn Song Drive, a quiet affluent tree-lined neighborhood of single-family homes.
FBI agents converge on the scene of the explosion
Police maintain a cordon near the site of an explosion in a southwest Austin residential area in on Sunday. Two male victims in their 20s were transported to South Austin Medical Center with serious injuries. 
A police vehicle blocks a road leading to the scene of an explosion in Austin on Sunday
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