The reported explosion on Tuesday evening that injured one man in a charity shop in Austin, Texas, was an incendiary device - not a package bomb, police say, BBC reports.
The authorities say it not is related to a series of recent parcel bombs in the area that have killed two people.
Instead an old military device donated to the store detonated.
The authorities have said the earlier blasts - involving package bombs and a tripwire - were all related.
President Donald Trump said "a very, very sick individual, or maybe individuals" carried out those blasts and pledged justice for the victims.
The city has been on edge after the attacks, and hundreds of police have been deployed to the area to investigate.