A charity shop employee was injured by an explosion in the city of Austin, where authorities are hunting a serial bomber.
The incident, on Tuesday evening local time, was not related to five other explosions in the area since 2 March, which have killed two people and seriously injured four others.
Police said an artillery simulator had been inside a box dropped off at a Goodwill charity shop in the city's south.
It had detonated, injuring the employee (a man in his 30s) and prompting a bomb scare.
Austin assistance police chief Ely Reyes said there was no reason to believe the incident was carried out by a copycat, adding that such military items were sometimes mistakenly donated to Goodwill instead of being disposed of properly.
The employee had been trying to dispose of the artillery simulator when it went off.
He was taken to hospital but has since been released.
Despite this, Sky News US Correspondent Greg Milam said the scare had shaken residents, as police continue to search for the person – or persons – responsible for the other explosions.
He said: "People in Austin are extremely on edge as a result of what's going on.
"This sense of alarm, sense of not knowing where they can go that's safe is pretty palpable at the moment."
Earlier on Tuesday, a FedEx worker was injured as a package containing nails and shrapnel blew up at a distribution centre in the San Antonio town of Schertz in the early hours of Tuesday morning.
The package had reportedly been bound for Austin.
Authorities intercepted another package at a FedEx facility near Austin airport a few hours later, which contained an explosive and was believed to be related to the other bombings.
On Sunday, two men, aged 22 and 23, were seriously injured in an explosion involving a tripwire device.
They have not been publicly named.
African-Americans Anthony Stephan House, 39, and Draylen Mason, 17, were the two men killed by the bombs.
Mr House died at his home in the first attack on 2 March, while Mr Mason was killed 10 days later.
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Hours after the attack on Mr Mason, a 75-year-old Hispanic woman was critically injured in a blast.
Police had been investigating the possibility that the bombings were racially-motivated but the tripwire in Sunday's incident appeared to be indiscriminate and the victims were both white men.