US President Donald Trump has said that a series of deadly bombings in Texas were carried out by "sick people", and also pledged justice for the victims.
"We're looking. It's not easy to find," Mr Trump said on Tuesday after a parcel bomb exploded at a FedEx depot - the fifth bomb to explode in recent days.
US authorities are probing whether that incident is connected to a series of bomb attacks targeting Austin.
Tuesday night's explosive was due to be shipped to that city, local police say.
"The bombings in Austin are terrible," Mr Trump told reporters before a meeting at the White House with Saudi Prince Mohammed bin Salman.
"This is obviously a very, very sick individual, or maybe individuals," he added.
"We will get to the bottom of it. We will be very strong."
Where was the most recent explosion?
The incident occurred at 00:30 (05:30 GMT) in Schertz, 65 miles (104km) south of the city of Austin, where four bombs have killed two people in recent weeks.
One person was lightly injured but did not require treatment, police said.
FBI spokeswoman Michelle Lee told US media that the agency believed the blast in Schertz could be connected to the previous attacks.
Local media, citing law enforcement sources, report that the bomb which went off in Schertz was addressed to Austin.
What else do we know of the FedEx explosion?
The bomb which exploded in Schertz was filled with nails and shrapnel, officials say.
A female employee standing nearby was treated at the scene for a possible concussion, the website adds. About 75 employees were working in the depot at the time.
Hours after the blast, police said a hazardous materials team had been sent to a FedEx shipping facility near Austin's airport to check reports of a suspicious package.
How did the earlier attacks unfold?
If the explosion in Schertz is linked to the previous four, it will be the first time the bomber has used the mail.
Three of the four devices were hidden in parcels left at residential addresses in Austin while police say the fourth was probably activated by a tripwire:
- The first device exploded on 2 March, killing Anthony Stephan House, 39, at his home
- Hours later on 13 March, a 75-year-old Hispanic woman, who has not been named, was injured by another package
- On 18 March, a device injured two men who may have set off a tripwire while walking along a street in the south-west of the city
Texas holding its breath
Gary O'Donoghue, BBC News, Austin
Five bombs in 18 days and still no real idea of motive.
That's the position the police and the FBI find themselves in as they analyse the latest blast, this time at a FedEx warehouse.
What seems clear is that they're dealing with someone or some group that has a significant level of knowledge of bomb making and who seem set on keeping the authorities guessing.
Even with $115,000 worth of reward on offer, there's still no sign of a promising lead.
There's understandable nervousness in Austin itself. The sheer geographical spread of the bombings making everyone feel vulnerable.
However, talking to some in the city last night, there was also a determination not to let the bomber get the upper hand.
"I decided I was still going to go out to dinner," one young woman told me.
But there was some frustration too. One drinker in a pub I met said he felt the police were being slow in tracking the bomber down.
Until they do, Texas is holding its breath, wondering where the next strike could come.
What lines of inquiry are police following?
Investigators initially said they were looking at a possible racial motivation for the attacks, which had killed two black men and injured a Hispanic woman.
However, the two men injured in the 18 March blast were both white.
"With this tripwire, this changes things," said Christopher Combs, the special agent in charge of the FBI's San Antonio office.
"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."
Fred Milanowski, the special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives in Houston, said the agency had a "high degree of confidence that the same individual built all these devices".
What action is being taken?
Mr Combs said more than 350 FBI special agents had been drafted in to work on the investigation, calling it an "unprecedented response."
Mr Trump praised the inter-agency response, but added: "We have to produce, we have to find this very sick person or people."
"We have to find them, really, immediately," he continued.
The FBI made a rare public appeal to the bomber to get in touch. "We would really like the bomber to contact us so we can talk to him," Mr Combs said.
Austin police have offered a $100,000 (£71,340) reward for information, and Texas Governor Greg Abbott has offered an additional $15,000.
Residents in Austin, a city of 974,000, have been asked to look out for suspicious packages.