Cops in the Texas city probed a parcel in a depot close to Austin's airport, and sealed off nearby roads.
How did the bombings start?
Austin, with a population of almost 1 million people, is home to the University of Texas and a plethora of technology companies and has been one of the fastest growing major USA cities.
The bombings have raised the question of whether the case can be more accurately seen as domestic terrorism or a hate crime - in part because the first victims of the bombs were people of color, including two black men who were killed.
Anthony Stephan House, 39, died from injuries he suffered after opening the package. The explosion at the FedEx center occurred sometime after midnight. All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores.
He also says that the explosion left what appeared to be nails embedded in his grandson's knees. The blast injured a 75-year-old woman who Manley said "came outside of her residence and found a package out front and picked up that package".
But Manley said police have been unable to determine a motive for the bombings. "We went out and heard all the sirens, but it was eerie".
Here is the timeline of events that have led hundreds of investigators on a manhunt.
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APD tweeted that it responded to 420 suspicious package calls between 8 a.m. Monday and 8 a.m. Tuesday, bringing the total to about 1,257 calls since March 12. It could have maimed any passer-by, police said.
"I will reach out to the suspect or suspects and ask that you contact us, ask that you reach out to us, communicate with us so that we can put this to an end", Manley said. It was triggered by a almost invisible tripwire, suggesting a "higher level of sophistication" than agents saw in three package bombs left on doorsteps, according to Fred Milanowski, agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. This is because the bomb was activated by a tripwire set alongside a roadway.
The fourth explosion, on Sunday, injured two white men.
Thad Holt, 76, said he is now watching his steps as he makes his way through a section of town near the latest attack. Christopher Combs of the FBI told reporters that the change in the bomb's mechanism meant a change in the investigation from a "more targeted approach to a more random approach".
Federal Bureau of Investigation special agent Michelle Lee said the explosion happened at around 1am local time at a FedEx facility in Schertz, just north-east of San Antonio and about 60 miles (95 kilometres) south-west of Austin.
Authorities say the package that exploded at a FedEx ground facility near San Antonio was on a conveyer belt when it detonated.
Police believe this bombing - the fourth since March 2 - is likely related to the other three incidents earlier this month, except this detonation used a trip wire. She was treated and released at the scene.
As a suspected serial bomber or bombers has the city of Austin, Texas on edge, authorities are scrambling to find out if a fifth explosion in the San Antonio suburb of Schertz is connected.
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