A package bomb that authorities believe is linked to the recent string of bombings in Austin, Texas, has exploded inside a FedEx distribution centre near San Antonio.

FBI 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.

One worker was treated for minor injuries and released, according to statements issued by the Schertz police department and FedEx.

An employee wrapped in a blanket talks to a police officer after she was evacuated at a FedEx distribution centre  (Eric Gay/AP)
An employee wrapped in a blanket talks to a police officer after she was evacuated at a FedEx distribution centre (Eric Gay/AP)

Ms 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 did not have details about the size, weight or description of the package.

The most recent bombing in Austin injured two men on Sunday night in the quiet neighbourhood of Travis Country in the south east of the city.

It was triggered on 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 federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

Sunday’s attack means the carnage by a suspected serial bomber who has terrorised Austin for weeks is now random, rather than targeted at someone in particular.

Investigators at the scene of a bomb explosion on Sunday in Austin (Jay Janner/Austin American-Statesman via AP)
Investigators at the scene of a bomb explosion on Sunday in Austin (Jay Janner/Austin American-Statesman via AP)

Authorities have not identified the two men injured on 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 remain in hospital in a stable condition.

Mr Grote said his grandson was conscious but was still in a lot of pain.

He said that on the night of the bombing, one of the victims was riding a bike in the street and the other was on a pavement 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.”

Mr Grote said his son, who lives about 100 yards from the blast, heard the explosion and raced outside.

“Both of them were kind of bleeding profusely,” Mr 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.

An agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives works with his dog near the site of Sunday's explosion (Eric Gay/AP)
An agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives works with his dog near the site of Sunday’s explosion (Eric Gay/AP)

“It’s creepy,” said Erin Mays, 33. “I’m not a scared person, but this feels very next-door-neighbour 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, the 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 dollars (£82,000).

“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.

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 motorway.

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.