A package bomb believed to be linked to the recent bombings in Austin has exploded inside a FedEx facility near San Antonio, while a second unexploded bomb was also found at the same facility, authorities said.

Key points: Latest attack comes after a string of bombings in Austin over the past two weeks

Latest attack comes after a string of bombings in Austin over the past two weeks FBI investigating explosion as being "possibly related" to package bomb investigation

FBI investigating explosion as being "possibly related" to package bomb investigation Investigators have been investigating possible race link

One employee was injured when the package containing nails and shrapnel exploded at the FedEx facility in Schertz, about 105 kilometres south of Austin, shortly after midnight on Tuesday, the local fire department said.

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The homemade bomb was bound for Austin, the site of the four other bombings.

Officials did not say if they believed the device was the work of a "serial bomber", who police feared may be responsible for the four earlier devices, which killed two people and injured six others.

The first three were parcel bombs left on residential doorsteps, while the fourth on Sunday was apparently set off by a trip wire.

Police warned that the latest bomb had a more sophisticated design than the others.

US President Donald Trump said "sick people" were behind the bombings in the city.

"The bombings in Austin are terrible ... This is obviously, a very, very sick individual or maybe individuals," he said.

"These are sick people and we will get to the bottom of it."

Fred Burton, chief security officer for Stratfor, a private intelligence and security consulting firm based in Austin, said the individual or people behind the bombings were likely to be highly skilled and methodical.

"This is a race against time to find him before he bombs again," he said.

Emergency vehicles outside the FedEx distribution centre where a package exploded. ( AP: Eric Gay )

Agents from the Federal Bureau of Investigation and the Bureau of Alcohol, Tobacco, Firearms and Explosives were on the scene and investigating, fire officials said.

They did not give the address for the package.

"We are investigating it as being possibly related to our open investigation," spokeswoman Michelle Lee told the Austin American-Statesman newspaper.

"We can't know for sure until we have an opportunity to look at the evidence itself."

Hours after the latest blast, police sent a hazardous-materials team to a FedEx facility in Austin to check on a suspicious package there. There was no immediate word about whether that package contained explosives.

Mayor seeks to reassure citizens

The wounded employee, who was not identified, was taken to a hospital with injuries that officials described as non life-threatening.

About 75 people were working at the facility at the time, fire officials said.

Police did not immediately say if the explosion appeared to be linked to the string of bombings in the Texas capital over the past two weeks.

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

FedEx officials could not be reached immediately for comment.

Further stoking fears, Austin police investigated a possible hazardous materials incident at a FedEx facility in Austin on Tuesday morning. There was no indication it was related to the bombings.

Austin mayor Steve Adler sought to reassure citizens on Tuesday.

"While the concern is real and the anxiety is real … it can't immobilise us. It has to make us more determined. It has to make us more vigilant," he told a local television station.

White House spokeswoman Sarah Sanders told Fox News they were monitoring the situation.

Sorry, this video has expired Texas police say tripwire was used to detonate Sunday's bomb

Investigators probe possible race link

The first two bombs killed black men and investigators believed that the third, which injured a Latina woman, may have been intended for a black family's home, police said earlier, raising the possibility they were a hate crime.

Sunday's trip wire bomb, which injured two white men, went off shortly after police made a rare public call to the suspect to explain his motives.

Parcel bombings in Austin earlier this month coincided with the start of the popular South By Southwest festival, prompting police to look at possible race links.

A package bomb exploded and killed a 17-year-old boy and injured a 40-year-old woman on March 12. They were both black.

Later that same day, about six kilometres away, another explosion injured the 75-year-old Hispanic woman.

This is the boarded-up doorway of the home that was hit with a parcel bomb on March 2. ( Reuters: Jon Herskovitz )

Those bombings are also believed to be linked to an attack that killed 39-year-old black man Anthony Stephan House on March 2.

"We cannot rule out that hate crime is at the core of this, but we are not saying that that is the cause," Police Chief Brian Manley told reporters after last week's bombings.

Reuters/AP