A package ‘containing nails and shrapnel’ addressed to Austin has exploded inside a FedEx facility in a suburb of San Antonio, Texas.
One member of staff suffered a mild injury in the explosion, which took place at a distribution center in Schertz, northeast of the city.
The package detonated as it was moving from one conveyor belt to another and the member of staff was not hit by the contents, local news reports.
Explosion: The package ‘containing nails and shrapnel’ detonated as it was moving from one conveyor belt to another at the FedEx sorting facility in a suburb of San Antonio, Texas
The package detonated just before 12.30am, and the female staffmember was treated for a headache, according to ABC7.
The package was destined for Austin, where four families have been targeted by the ‘serial bomber’ in recent weeks which have left two dead and four injured.
The latest blast occurred at around 8.30pm on Sunday in a southwestern Austin residential neighborhood known as Travis Country.
The explosion seriously injured two white men, aged 22 and 23, when they are believed to have triggered a tripwire when they were pushing or riding their bikes down a sidewalk.
An FBI agent and Austin cops work together at the scene of an explosion on Sunday night
Previous victims: Anthony Stephan House, 39, (left) died on March 2 and Draylen Mason, 17, (right) died on March 12 when package bombs were left at their respective homes
This map shows the latest explosion along with the three prior package bombings in March
It 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.
Austin Police Chief Brian Manley said in a Monday morning press conference they are dealing with a ‘serial bomber’ and that the latest attack showed a ‘higher level of skill’ than the three previous bombings.
Police also said they have still been unable to determine a motive for the string of bombings which have killed two people in Austin and put the city of nearly one million on edge.
‘We’re clearly dealing with what we expect to be a serial bomber at this point based on the similarities between now what is the fourth device,’ the police chief said.
‘(But) what we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night an attack that would have hit a random victim that happened to walk by.’
‘So we’ve definitely seen a change in the method that this suspect or suspects are using.’
In the earlier bombings, two African-American men were killed by packages left on their doorsteps, raising the possibility of a hate crime. A 75-year-old Hispanic woman was also injured in a blast.
Manley said the latest attack, which injured the two white males, appeared ‘random’ and was triggered by a tripwire.
He added that it was an escalation from the package bombs that had targeted the previous victims.
‘A trip wire doesn’t necessarily suggest a military background,’ Manley said.
Sniffer dogs and authorities canvased the area on Monday morning after a bomb exploded in the Travis Country neighborhood on Sunday night
Bomb site: A package bomb exploded at this home in northeast Austin, killing Anthony House
Another bombing: A package explosion in the kitchen of this home on March 12, ten days later, killed Draylen Mason, 17
Same day explosion: A package bomb, also on March 12, seriously wounded Esperanza Herrera, 75
‘But it suggests that the suspect or suspects we are dealing with have a higher level of sophistication than we believed, as they’re changing their methods to a more difficult device.’
The two men injured in the latest blast where white men, while the two men killed were black.
The first was a package bomb that exploded at a northeast Austin home on March 2, killing a 39-year-old Anthony Stephan House.
Two more package bombs then exploded farther south on March 12. Draylen Mason, 17, was killed and his mother was wounded after they opened a package in their kitchen.
A 75-year-old Hispanic woman named by family as Esperanza Herrera was severely injured when a package bomb exploded at her home a few hours later.
The families of the two men who died knew each other and both men were involved in activism in the black community. It led to speculation of a racial motive in the attacks but that has not been confirmed by investigators.