Police discovered another package at the same location that they believe is also loaded with an explosive device, San Antonio Police Chief Bill McManus told reporters.
One worker was injured after the package destined for Austin, exploded at the facility on 9935 Doerr Lane after midnight, police said.
Several other packages have exploded recently in Austin, Texas.
Investigators have found similarities between that device and three previous bombs, which were stuffed inside packages and left on residents' doorsteps, Austin Police Chief Brian Manley said. The tripwire set off the device, which was left on the side of a residential road near a fence, at 8:32 p.m., Sunday, police said.
The systems can be used at the scene so they can quickly assess the safety of packages. On the night of the bombing, one of the victims was riding a bike in the street and the other was on a sidewalk when they crossed a tripwire that he said knocked "them both off their feet".
"The FBI is now investigating a confirmed link between packages involved in the Austin bombing investigation and a mail delivery office in Sunset Valley". Police say the men may have triggered a tripwire next to a fence.
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.
"It would be silly for us not to admit that we suspect it's related" to the four bombings in Austin this month, FBI San Antonio spokeswoman Michelle Lee said.
"The Austin Police Department is aware of the incident that has occurred in Schertz, Texas, and is working closely on the investigation with our federal partners", he tweeted. "When I heard it was a bomb, I was really distraught, because it was one of my friends, one of our neighbors that I've known for a really long time", said neighbor Walker Weeden.
The blast follows a Sunday night blast that was triggered along a street by a almost invisible tripwire, suggesting a "higher level of sophistication" than agents saw in three early package bombs left on doorsteps.
"I'm scared about what's going to happen next", she said.
Manley told residents not to touch or go near anything that looks suspicious.
Hansen said it was not clear where the package was being sent.
The earlier blasts in Austin were being probed as possible hate crimes.
Police have urged Austin residents to report any suspicious packages.
Shortly before the fourth bombing, the reward for information leading to the arrest of anyone responsible for the three blasts increased to a total of $115,000, authorities said. Underscoring that point, a relative says the most-recent explosion left what appeared to be nails stuck in his grandson's knees.
"If this bomber is being motivated, in part, by the sense of power and control that he has holding the city of Austin in a state of fear, and depending how addictive that feeling is", she said, "that can be a strong contributor to his doing it again and not waiting a long period of time".