A package bomb containing nails and shrapnel that was destined for an address in Austin, Texas, exploded at a FedEx Corp distribution centre in San Antonio, injuring one employee, San Antonio fire officials said.
That blast came two days after the latest of four powerful homemade bombs set off in Austin this month that have killed two people and injured six more in what police warn appears to be the work of a serial bomber.
Hundreds of investigators have joined local authorities in probing the bombings, which the White House said do not appear to be linked to terrorism.
The latest blast knocked a female employee off her feet, police said.
The package exploded shortly after midnight local time at the FedEx Corp sorting facility in Schertz, Texas, 105km south of Austin, the San Antonio Fire Department said on Twitter.
Investigators were also examining a second suspicious package at the same facility, San Antonio Police Chief Bill McManus told reporters.
"There was one other package that we believe was also loaded with an explosive device that they are looking at right now," Mr McManus told reporters in Schertz.
In Sunset Valley, a town within Austin, police surrounded a FedEx store on suspicion it was linked to the Schertz bombing.
"The FBI is currently investigating a confirmed link between packages involved in the Austin bombing investigation and a mail delivery office in Sunset Valley. It appears that the source of the suspect packages was a private package delivery office in Sunset Valley," Sunset Valley police said in a statement.
Even with the apparent Austin link, it remained unknown whether the latest incident was the work of what Austin police said could be a serial bomber who is responsible for the four earlier devices that killed two people and injured four others.
The blast at the FedEx facility in Schertz was the fifth in the state in the last 18 days and the first involving a commercial parcel service.
President Donald Trump vowed that the authorities will get to the bottom of the bombings, saying whoever carried them out was "very, very sick".
Speaking at the White House, he said: "It's terrible. This is obviously a very, very sick individual or maybe individuals.
"These are sick people, and we will get to the bottom of it."
Investigators were trying to come up with a theory or intelligence regarding the motive for the bombings or identity of the bomber or bombers, a US security official and a law enforcement official told Reuters.
The FBI was investigating the FedEx package explosion as if there were a connection to the Austin bombings, the law enforcement official said. Both sources declined to be identified.
