A parcel bomb has exploded at a FedEx depot near San Antonio, Texas, injuring one, US news reports say.
The incident reportedly occurred overnight in the town of Schertz, an hour south of Austin where four bombs have been planted in recent weeks.
The Washington Post reported that police were investigating possible links to the previous devices, which killed two people.
Authorities believe the previous blasts are the work of a serial bomber.
Three of the four previous devices planted in Austin were hidden in parcels left at residential addresses. Police said another was probably activated by a tripwire, triggered by two men walking along a street in south-west Austin.
The first exploded on 2 March, killing Anthony Stephan House, 29, at his home.
Two more bombs exploded 10 days later. Draylen William Mason, 17, was killed and his mother was critically injured when he brought a package inside his home.
Hours later, a 75-year-old Hispanic woman, who has not been named, was injured by another package.
Investigators initially said that they were looking at a possible racial motivation for the attacks, which killed two black men and injured a Hispanic woman, but the tripwire appeared to be indiscriminate and was triggered by two white men.
“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.”
Image copyright FBI
Mr Combs said more than 350 special agents were working on the investigation, calling it an “unprecedented response.”
Austin police have offered a $100,000 (712,000) reward for information. Texas Governor Greg Abbott has offered an additional $15,000.
