(FOX NEWS) - AUSTIN, Texas (AP) -- A package bomb that authorities believe is linked to the recent string of Austin bombings exploded early Tuesday inside of a FedEx distribution center near San Antonio, leaving one worker with minor injuries.
Hours later, police sent a hazardous materials team to a FedEx facility in Austin to check on a suspicious package there. There was no immediate word about whether that package contained a bomb.
FBI agent Michelle Lee said the explosion happened at around 1 a.m. at a FedEx facility in Schertz, which is just northeast of San Antonio and about 60 miles (95 kilometers) southwest of Austin. One worker was treated for minor injuries and released, according to statements issued by the Schertz Police Department and FedEx.
Lee said that although it is still early in the investigation, "it would be silly for us not to admit that we suspect it's related" to the four Austin bombings that have killed two people and injured four others since March 2. She didn't have details about the size, weight or description of the package.
The most recent bombing in Austin injured two men Sunday night in the quiet neighborhood of Travis Country in the southeast of the city. It was triggered along a street by a nearly invisible tripwire, suggesting a "higher level of sophistication" than agents saw in three early 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.
Sunday's attack means the carnage by a suspected serial bomber that has terrorized Austin for weeks is now random, rather than targeted at someone in particular.
Authorities haven't identified the two men injured Sunday, saying only that they are in their 20s and white. But William Grote told The Associated Press on Monday that his grandson was one of them and that he had what appeared to be nails embedded in his knees. Police described the men's injuries as significant, and both remained hospitalized in stable condition on Monday.
Grote said his grandson was cognizant but was still in a lot of pain. He said 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."
"It was so dark they couldn't tell and they tripped," he said. "They didn't see it. It was a wire. And it blew up."
Grote said his son, who lives about 100 yards (90 meters) from the blast, heard the explosion and raced outside. "Both of them were kind of bleeding profusely," Grote said.
That was a departure from the first three bombings, which involved parcels left on doorsteps that detonated when moved or opened.
The tripwire twist heightened the fear around Austin, a town famous for its cool, hipster attitude.
"It's creepy," said Erin Mays, 33. "I'm not a scared person, but this feels very next-door-neighbor kind of stuff."
Authorities repeated prior warnings about not touching unexpected packages and also issued new ones to be wary of any stray object left in public, especially ones with protruding wires.
"We're very concerned that with tripwires, a child could be walking down a sidewalk and hit something," Christopher Combs, FBI agent in charge of the bureau's San Antonio division, said in an interview.
Police originally pointed to possible hate crimes, but the victims have now been black, Hispanic and white and from different parts of the increasingly diverse city. Domestic terrorism is among the variety of possible motives investigators are looking at.
Local and state police and hundreds of federal agents are investigating, and the reward for information leading to an arrest has climbed to $115,000.
"We are clearly dealing with what we believe to be a serial bomber at this point," Austin police Chief Brian Manley said, citing similarities among the four bombs. He would not elaborate, though, saying he didn't want to undermine the investigation.
While the first three bombings all occurred east of Interstate 35, a section of town that tends to be more heavily minority and less affluent, Sunday's was west of the highway. The differences in where the blasts have occurred, the lack of a motive and other unknowns make it harder to draw conclusions about a possible pattern, further unnerving a city on edge.
Thad Holt, 76, said he is now watching his steps as he makes his way through a section of town near the latest attack. "I think everybody can now say, 'Oh, that's like my neighborhood,'" he said.
The ATF's Milanowski said the latest bomb was anchored to a metal yard sign near the head of a hiking trail.
"It was a thin wire or filament, kind of like fishing line," he said. "It would have been very difficult for someone to see."
Milanowski said authorities have checked more than 500 leads. Police asked anyone with surveillance cameras at their homes to come forward with the footage on the chance it captured suspicious vehicles or people.