AUSTIN, Texas >> Investigators pursuing a suspected serial bomber in the Texas capital shifted their attention today to a FedEx shipping center near San Antonio where a package exploded on a conveyor belt in the middle of the night and caused minor injuries to a worker.

Although the latest blast did not inflict serious harm, it added to the widening fear of more strikes like those that have already killed two people and badly wounded four others.

Hours after the explosion, police sent a bomb squad to a FedEx facility outside Austin’s main airport to check on a suspicious package that was reported shortly before sunrise. There was no immediate word about whether that package contained explosives.

Investigators also closed off an Austin-area FedEx store where they believe the bomb was sent to the distribution center. Authorities roped off a large area around the shopping center in the enclave of Sunset Valley.

FBI agent Michelle Lee said the explosion happened around 1 a.m. at a FedEx facility in Schertz, which is just northeast of San Antonio and about 60 miles southwest of Austin.

“It would be silly for us not to admit that we suspect it’s related” to the other four Austin bombings since March 2, Lee said. She did not have details about the size, weight or description of the package.

One worker reported ringing in her ears. She was treated at the scene.

Before it exploded, the package had been sent from Austin and was addressed to a home in Austin, Texas Attorney General Ken Paxton said.

Police initially reported finding an unexploded parcel at the same FedEx facility, but later said they were mistaken and that the only bomb found was the one that exploded.

The FedEx blast came less than two days after another bombing wounded two men Sunday night in a quiet Austin neighborhood. It was triggered by a nearly invisible tripwire, suggesting a “higher level of sophistication” than agents saw in three package bombs left on doorsteps, according to Fred Milanowski, the agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

A criminologist at the University of Alabama said if a single perpetrator is behind the blasts, changing the means of delivery increases the bomber’s chance of getting caught.

“I think it would suggest that the bomber is trying to stay unpredictable,” Adam Lankford said. “But it also increases the likelihood that he would make a mistake.”

Authorities have not identified the two men who were hurt 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.

Grote said his grandson was in a lot of pain. 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 from the blast, heard the explosion and raced outside to find both of the young men bleeding profusely.

The presence of a tripwire was a departure from the first three bombings, which involved parcels left on doorsteps that detonated when moved or opened.

The tripwire heightened fears around Austin, a town famous for its cool, hipster attitude.

In Washington, President Donald Trump said the assailant behind the bombing is “very sick.”

During an Oval Office meeting today with Saudi Arabia’s Crown Prince Mohammed bin Salman, Trump described the situation as “terrible.”

“This is obviously a very sick individual or individuals,” and authorities are “working to get to the bottom of it.”

Police repeated prior warnings about not touching unexpected packages and 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 city.

“We are clearly dealing with what we believe to be a serial bomber,” Austin Police Chief Brian Manley said, citing similarities among the bombs. He would not elaborate, saying he did not 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 attack was west of the highway. The differences in location, the lack of a motive and other unknowns make it harder to draw conclusions about any possible pattern.

Thad Holt 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 latest bomb was anchored to a metal yard sign near the head of a hiking trail, Milanowski said.

“It was a thin wire or filament, kind of like fishing line,” he said. “It would have been very difficult for someone to see.”

Police asked anyone with surveillance cameras at home to come forward with the footage on the chance it captured suspicious vehicles or people.