A package believed to be bound for Austin exploded at a FedEx facility about an hour south of that city, expanding the investigation into the bombings that have rattled the Texas capital this month.
Police have said the bombing campaign in Austin is the work of a skilled bomber or bombers, and they have expressed concerns that whoever is responsible has recently shifted tactics - moving from leaving package bombs outside homes to setting a device rigged with a tripwire - showing even greater sophistication than investigators first believed.
The explosion occurred while the device was on an automated conveyor belt at a FedEx centre in Schertz, Texas, just outside San Antonio. One employee said it caused ringing in her ears, but no one else was affected, police said.
Police have said they believe the explosions in Austin are the work of a "serial bomber." While authorities have not conclusively linked the explosion in Schertz to the bombings in Austin, officials say they believe these blasts are all the work of the same person or people.
Michael Hansen, the Schertz police chief, said investigators were "confident that neither this facility nor any location in the Schertz area was the target."
The explosion in Schertz opened up a new front in the investigation into the bombings that have killed two people, injured four others and sent waves of uncertainty and fear across the Austin region.
William McManus, the San Antonio police chief, had said that a second, unexploded device was also found at the Schertz facility. However, a spokesman for his department backtracked later today, saying that McManus "misspoke" when making those comments.
During a news briefing, McManus had said officials believed a second package found at the Schertz facility was "also loaded with an explosive device that they are working on right now." A spokesman later issued a statement saying: "There is no secondary device at the Schertz facility."
The Schertz police did not immediately respond to a message seeking comment.
FedEx, in a statement, said it had "confirmed that the individual responsible also shipped a second package that has now been secured and turned over to law enforcement."
The company also suggested it had found evidence related to whoever shipped the packages and given that to investigators.
"We have provided law enforcement responsible for this investigation extensive evidence related to these packages and the individual that shipped them collected from our advanced technology security systems," the FedEx statement said.
The confusion underscored the frantic pace of the investigation, which has widened after five explosions in less than three weeks and now includes more than 300 federal agents along with scores of local law enforcement officers in Texas.
Investigators also searched a FedEx facility in Austin where the package that detonated in Schertz would likely have been sent next, looking to see if they could find other, similar package bombs there, according to one person familiar with the investigation.
An Austin police spokeswoman said that authorities were responding to a suspicious package at the FedEx facility there, though she said it was not immediately clear if this was connected to the explosion in Schertz.
Any additional unexploded device could be crucial to the expanding investigation into the string of blasts that have shaken Austin in recent weeks, because a bomb that has not detonated could provide significant evidence to help authorities identify a suspect or suspects. In bomb investigations, unexploded devices can be critical to narrowing the search for suspects because the materials used to assemble the device can be traced back to the supplier - and, in many cases, the individual purchaser.
Brian Manley, the interim Austin police chief, said his department had sent officers to Schertz to participate in the investigation there.
"The working theory right now . . . is that that was a package that was in the shipping center destined for Austin," he told the Austin City Council.
Manley said the explosion in Schertz "brought in a new development" in the case, which he said has created new concerns in the city. All four explosions in Austin so far have occurred outside the city's core, which includes downtown, the Texas capitol and the University of Texas.
The first three bombs - one on March 2 and a pair that detonated March 12 - were in packages delivered to people's homes, authorities said. But the fourth bomb was placed on the side of the road in a residential neighbourhood and rigged with a tripwire, so when it detonated, two men walking through the area were wounded.
"That bomb was very different from the other three," Manley told the City Council. "The first three appeared to be targeting a specific residence, resident address. And whether they were targeting the person at that address or not, we know they were placed on a specific doorstep at a specific home."
The device on Monday, by contrast, was seemingly intended to detonate at random, ratcheting up the fear in a city already unnerved by the package explosions. Police said today that they had received more than 1200 calls about suspicious packages since March 13 NZT, the day two bombs detonated; a third of these calls came in between yesterday and today.
"With this tripwire, this changes things," Christopher Combs, special agent in charge of the FBI's San Antonio office, said. "It's more sophisticated. It's not targeted to individuals. We're very concerned that with tripwires, a child could be walking down the sidewalk and hit something."
Authorities have seemed at a loss to explain who could be setting off the devices - or why - and have pleaded with the unknown attacker to communicate with them. They also have offered rewards of up to US$115,000 for information in the case.
The first three explosives all detonated in eastern Austin, affecting areas where black and Hispanic residents live, leading to suggestions that they may have been motivated by racial bias. The explosion on Monday, though, detonated in an affluent, predominantly white neighbourhood and injured two white men in their 20s.
"This is a public threat," Nelson Linder, president of NAACP's Austin chapter, said. "Now that the geography has changed, it's going to widen people's perspectives. Nobody can take this lightly; we're all vulnerable."
Linder added: "Like they tell us in the military, when you walk, look down at where you're walking."
President Donald Trump said today that the federal Government is working "hand in hand" with local authorities to "get to the bottom" of the bombings in Austin and find those responsible.
"This is obviously a very, very sick individual, or maybe individuals," he told reporters in the Oval Office. "These are sick people, and we will get to the bottom of it, and we will be very strong. We have all sorts of federal agencies over there right now. We're searching. What's going on in Austin - a great place, tremendous place - is absolutely disgraceful."
Dan Defenbaugh, a former FBI official who spent decades investigating bomb cases, expressed concern that the bomber or bombers had apparently shifted their tactics.
"Once a bomb builder makes a device, they usually make it the same way each and every time," said Defenbaugh, who managed the Oklahoma City bombing investigation. "That's not happening here. I'm also troubled by the fact that there hasn't been an extortion demand or communication."
Authorities have flooded central Texas with hundreds of investigators, though Defenbaugh noted that they will have to closely coordinate due to volume of tips and information in such cases.
"I'm sure it's on the minds of a lot of law enforcement that they have a serial killer on their hands," he said.