Texas has been hit by its fifth explosion this month as police hunt a suspected serial bomber.
A FedEx worker was injured after a package containing nails and shrapnel blew up at a distribution centre in the San Antonio town of Schertz.
The package had reportedly been bound for Austin – the state capital where a series of blasts in March has so far killed two people.
On Sunday, two men, aged 22 and 23, were seriously injured in an explosion involving a tripwire device.
The FedEx employee is believed to have suffered non-life threatening injuries in the latest incident, which happened at about midnight local time.
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are investigating.
Federal agents say the package is likely to be linked to attacks by a suspected serial bomber.
The string of bombings in Austin over the past two weeks have killed African-American men Anthony Stephan House, 39, and Draylen Mason, 17, and injured six others.
Mr House died at his home in the first attack on 2 March, while Mr Mason was killed 10 days later.
Hours after the attack on the teenager, a 75-year-old Hispanic woman was critically injured in a blast.
There is the possibility that the explosions are racially motivated crimes but the tripwire appeared to be indiscriminate and was triggered by two white men.
The first three devices were parcel bombs dropped off at night in front of homes in the east side of the Texas capital.
The fourth incident involving a tripwire device went off in a west side neighbourhood of Austin on Sunday.
Police have said all four devices were similar and more than 500 FBI agents are helping in the search for the culprit.
Before the latest explosion, police said the bombings were being looked at as possible hate crimes and that the attacker appeared to be trying to send a message.
Austin police chief Brian Manley said: “We are not going to understand that (message) until the suspect or suspects reach out to us to talk to us about what that message was.”
The National Association for the Advancement of Coloured People has described the bombs as domestic terrorism.