A package 'containing nails and pieces of metal' destined for Austin has exploded and injured one person inside a FedEx facility in nearby San Antonio in the fifth bombing to rock the state this month.

The wounded employee was taken to hospital after suffering a mild injury in the explosion at the distribution center in Schertz, about 65 miles south of Austin, shortly after midnight.

The package detonated as it was moving from one conveyor belt to another and the female staff member, who was not hit by the contents, was treated for a headache, possibly caused by a concussion. 

Federal agents say the package is likely linked to four other attacks this month in Austin, some 80 miles north-east of San Antonio, that have left two dead and four injured.

Authorities say a 'serial bomber' is at large and have warned that the devices appear to be getting more sophisticated. 

Just hours after the package explosion in San Antonio, emergency crews were called to a FedEx facility in Austin following reports of a suspicious package. There were no immediate details available about that incident.