Emergency teams have rushed to another reported explosion in Austin - this one at a Goodwill store - but authorities say the blast isn't related to recent bombings that killed and injured people and caused panic across the Texas capital.
Police and emergency response teams said an "incendiary device" exploded late on Tuesday, injuring a man in his 30s.
Nearby stores, shopping centres and restaurants were evacuated.
But police and the US Bureau of Alcohol, Tobacco, Firearms and Explosives said a short time later that it was unrelated to the previous blasts.
Gary Davis, president and CEO of Goodwill Texas, stood outside a police barrier huddling with other Goodwill employees. He said the device was contained in a bag and detonated when a worker moved it.
"We put all the donations we get in a big cardboard box. He pulled something out in a bag, completely normal, and the device went off," Davis said.
"In this town, if an incendiary device goes off, everybody just scatters and panics. We're all on edge."
The incident came as investigators pursing a suspected serial bomber terrorising Austin for weeks uncovered what seemed like valuable new leads.
Even before the report of Tuesday night's explosion, it had already been a busy day.
Before dawn, a bomb inside a package exploded around 1am as it passed along a conveyer belt at a FedEx shipping centre near San Antonio, causing minor injuries to a worker.
The Austin Police Department, the FBI and other federal agencies confirmed that the package centre blast was related to four previous ones that killed two people and seriously injured four others.
That explosion occurred at a FedEx facility in Schertz, northeast of San Antonio and about 95km southwest of Austin.
Later in the morning, police sent a bomb squad to a FedEx facility outside the Austin airport to check on a suspicious package that was reported.
Federal agencies and police later said that package had indeed contained an explosive that was successfully intercepted and that it, too, was tied to the other bombings.
Authorities also closed off an Austin-area FedEx store where they believe the bomb that exploded was shipped to the distribution centre.
Michael McCaul, a Republican from Austin who is chairman of the House Homeland Security Committee, said that investigators have obtained surveillance videos that "could possibly" show a suspect, but are still poring through video.
"I hope his biggest mistake was going through FedEx," McCaul said of the bomber in a phone interview.
He added that the person responsible for the bombings had previously been "very sophisticated in going around surveillance cameras".
"They've got a couple of videos that could possibly be the person but they're not sure at this point," McCaul said.
Before it exploded, the package had been sent from Austin and was addressed to a home in Austin, Texas Attorney General Ken Paxton said.
In a statement, FedEx officials said the same person responsible for sending the package also shipped a second parcel that has been secured and turned over to law enforcement.
The Schertz blast came less than two days after a bombing wounded two men Sunday night in a quiet Austin neighbourhood 5km from the FedEx store.
It was triggered by a nearly invisible tripwire, suggesting a "higher level of sophistication" than agents saw in three package bombs previously left on doorsteps, according to Fred Milanowski, the agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
In Washington, President Donald Trump said the assailant behind the bombing was "very sick".
"This is obviously a very sick individual or individuals", and authorities are "working to get to the bottom of it", he said.
Australian Associated Press