A package bomb exploded at a FedEx distribution centre lightly wounding one worker near San Antonio, Texas on Tuesday after a series of blasts in the state capital of Austin this month.
Officials did not say if the latest attack was the work of what Austin police believe could be a serial bomber responsible for the four earlier explosive devices, which killed two people and wounded four others.
The explosion at the FedEx facility in Schertz, northeast of San Antonio, was the fifth in the state in the last 18 days. If it is linked to the others, it would be the first outside the Austin area and the first involving a commercial parcel service.
"We are investigating it as being possibly related to our open investigation," FBI spokeswoman Michelle Lee told the Austin American-Statesman newspaper. "We can't know for sure until we have an opportunity to look at the evidence itself."
The package, filled with nails and metal shrapnel, exploded shortly after midnight local time (05:00 GMT) at the facility, about 105km south of Austin.
The individual or people behind the bombings are likely to be highly skilled and methodical, said Fred Burton, chief security officer for Stratfor, a private intelligence and security consulting firm based in Austin.
"This is a race against time to find him before he bombs again," Burton said.
More than 500 federal agents were involved in the investigation.
Targeting Texas
The first three devices were parcel bombs dropped off in front of homes in three eastern Austin neighbourhoods. The fourth went off on Sunday night on the west side of the city and was described by police as a more sophisticated device detonated through a trip-wire mechanism.
The four bombs were similar in construction, suggesting they were the work of the same bomb-maker.
"We're very concerned that with tripwires, a child could be walking down a sidewalk and hit something," Christopher Combs, FBI agent in charge of the bureau's San Antonio division, told The Associated Press.
Police originally pointed to possible hate crimes as a motive for the attacks, but the victims have now been black, Hispanic and white and from different parts of Austin.
The spate of bombings in the Texas capital of Austin have no known links to "terrorism", White House spokesman Sarah Sanders said on Tuesday.
"We are committed to bringing perpetrators of these heinous acts to justice. There is no apparent nexus to terrorism at this time," Sanders said in a Twitter post.
