The latest cluster of booby-trapped packages brought to six the number of explosive devices that have come under investigation in Texas this month.
Baffled investigators have taken the extraordinary step of making public appeals asking that whoever is responsible to at least come forward with a demand or an explanation.
Early on Tuesday, a package filled with nails and metal shrapnel exploded at about 12.30am (local time) on a conveyer belt at FedEx sorting centre in Schertz, near San Antonio, knocking a female employee off her feet, officials said.
The package was being sent from Austin to another address in Austin and passed through a sorting center in Schertz, about 105km away.
Authorities said the worker was treated for her injuries at the scene.
Later in the morning, authorities were alerted to a suspicious package at a FedEx facility in Austin.
Police and federal agents called to the scene found the package contained a bomb and it "was disrupted by law enforcement," according to a joint statement from Austin police, the Federal Bureau of Investigation and the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).
No injuries were reported, the statement said.
The three agencies said investigators had determined that the two FedEx bombs "are connected" to the four package explosions that occurred between March 2 and March 18 in Austin, killing two people and injuring four others.
But officials with the ATF said the incident Tuesday evening in south Austin, which seriously injured a man in his 20s, "does not appear to be related" to the previous incidents. Local emergency services officials said the victim was expected to survive.
Speaking through the media, officials have appealed to the bomber to reveal the motives for the attacks. They have also asked the public for any tips, offering a $US115,000 ($NZ160,000) reward.
"Somebody has to know something," FBI spokeswoman Christina Garza said. "The person behind these explosives, please, we want to know why."
"This is obviously a very, very sick individual, or maybe individuals," President Donald Trump told reporters. "These are sick people, and we will get to the bottom of it."
