Emergency teams are responding to another reported explosion in Austin, Texas, this one at a Goodwill store in the southern part of the city.

In a tweet, the Austin Police Department urged residents to avoid the area.

Austin-Travis County EMS said there had been reports of at least one person injured.

It came as investigators who have pursued a suspected serial bomber terrorising Austin for weeks uncovered what seemed like valuable new leads in the case.

Even before the report of the Goodwill blast, it had already been a busy day for authorities.

Before dawn Tuesday, 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, just northeast of San Antonio and about 95 kilometres 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.

Federal agencies and police later said that package had indeed contained an explosive that was successfully intercepted by authorities. They added that the intercepted package, too, was believed to be related to the other bombings.

Meanwhile, 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, who has spoken to federal investigators and Austin police Chief Brian Manley, 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 about 5 kilometres 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.

In Washington, President Donald Trump said the assailant behind the bombing is "very sick."

This is obviously a very sick individual or individuals," and authorities are "working to get to the bottom of it", he said.