EMERGENCY personnel responded to another reported explosion in Austin, Texas, this time at a Goodwill store in the southern part of the city.

But police said an incendiary device, not a package bomb, was what exploded at the Goodwill store, and it wasn’t related to prior bombings.

At least one person was injured but details about the severity of those injuries and the explosion itself were unknown.

It was the sixth explosion in the Austin area since March 2. So far, two people have been killed and four others seriously wounded.

It came after a bomb containing nails and shrapnel that was destined for an address in Austin, Texas, exploded at a FedEx Corp distribution centre in San Antonio, injuring one employee.

That blast came two days after the latest of four powerful homemade bombs set off in Austin this month, which killed two people and injured six more in what police warned appeared to be the work of a serial bomber.

The package exploded shortly after midnight local time at a distribution facility in Schertz, Texas, outside of San Antonio, about 105km south of Austin, the San Antonio Fire Department said on Twitter.

The package was on a conveyer belt when it detonated. One worker reported ringing in her ears after the blast. She was treated and released from a hospital.

Texas Attorney General Ken Paxton told KXAN that the package was being mailed from Austin back to Austin.

FBI agent Michelle Lee said “it would be silly for us not to admit that we suspect it’s related” to the four Austin bombings that have killed two people and injured four others since March 2.

The first three were parcel bombs dropped off in front of homes on Austin’s east side, with the fourth an apparent trip-wire device that went off on the city’s west side on Sunday.

William Grote told The Associated Press that his grandson was one of two men who were injured on Sunday and that he had what appeared to be nails embedded in his knees.

On the night of the bombing, one of the victims was riding a bike in the street and the other was on a sidewalk when they crossed a tripwire that he said knocked “them both off their feet.”

“It was so dark they couldn’t tell, and they tripped,” Mr Grote said. “They didn’t see it. It was a wire. And it blew up.”

Mr Grote said his son, who lives about 90 metres from the blast, heard the explosion and raced outside to find both of the young men bleeding profusely.

The first four devices were similar in construction, suggesting they were the work of the same bomb maker, officials said.

Anthony Stephan House, 39, died on March 2 and Draylen Mason, 17, died on March 12 when package bombs were left at their respective homes.

Police originally pointed to possible hate crimes, but the victims have now been black, Hispanic and white and from different parts of the city.

US President Donald Trump, who had been criticised for not responding to the terror facing Austin, finally commented on the attacks.

“The bombings in Austin are terrible,” Mr Trump said. “Local, state, federal are working hand-in-hand to get to the bottom of it. This is obviously a very, very sick individual - and maybe individuals. These are sick people, and we will get to the bottom of it.”