A package bomb believed to be linked to several recent blasts in Austin, Texas, has exploded inside a FedEx distribution centre near San Antonio.
Hours later, police sent a hazardous materials team to a FedEx facility in Austin to check on a suspicious package there.
FBI agent Michelle Lee said the explosion happened at around 1am local time at a FedEx facility in Schertz, which is just north east of San Antonio and about 60 miles south west of Austin.
A worker was treated for minor injuries and released, police said.
“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, Ms Lee said.
The package was on a conveyor belt when it detonated. One worker reported ringing in her ears after the blast. She was treated and released from hospital.
Texas attorney general Ken Paxton said the package was sent from Austin and was addressed to a home in the city.
Mr Paxton also told television station KXAN that a second parcel bomb that did not explode was found at the FedEx facility in Schertz.
San Antonio police chief William McManus told a news conference that the second package was “no longer” at the Schertz facility.
The FedEx blast came less than two days after another bombing wounded two men on Sunday night in a quiet Austin neighbourhood.
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, agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Authorities have not identified the two men who were hurt on Sunday, saying only that they are in their 20s and white.
But William Grote told the Associated Press 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 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 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 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.
“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 issued new ones to be wary of any stray objects 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 city.
Local and state police and hundreds of federal agents are investigating. 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 location, the lack of a motive and other unknowns make it harder to draw conclusions about any possible pattern.