A parcel bomb, bound for Austin, Texas, exploded at a FedEx distribution depot in the US state, early on Tuesday.
It follows a string of bombings over the past two weeks in the city, that have killed two people and wounded four.
The package, containing nails and shrapnel, exploded shortly after midnight local time at a distribution facility in Schertz about 105 km south of Austin, the San Antonio Fire Department said on Twitter.
Initial reports said a wounded employee had been taken to hospital with injuries described as non life-threatening.
However the Bureau of Alcohol, Tobacco, Firearms and Explosives later said there had been no injuries from the blast.
It is unclear whether this latest explosion is related to the others.
The FBI has sent hundreds of special agents to Austin where authorities say a "serial bomber" is at large.
The first three devices were parcel bombs left on residential doorsteps, while the fourth on Sunday was apparently set off by a trip wire.
The first two bombs killed black men and investigators believed that the third, which injured a Latina woman, may have been intended for a black family's home, police said, raising the possibility they were a hate crime.
Sunday's trip wire bomb, which injured two white men, went off shortly after police made a rare public call to the suspect to explain his motives.