A package bound for Austin, Texas, has blown up at a FedEx distribution centre near San Antonio, reportedly injuring a female worker.

The blast came two days after a fourth powerful homemade bomb was set off in Austin. Two people have been killed and six more injured in what appears to be the work of a serial bomber.

The latest device exploded shortly after midnight on Tuesday morning at a building in Schertz, a city 65 miles south of Austin, officials said.

There were unconfirmed reports the package contained nails and shrapnel. One employee apparently suffered a "percussion-type" injury from the blast.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) were at the scene, with officials saying the package is likely linked to a string of explosions in Austin.

On Monday, Austin police said they were looking for a suspect after the fourth package bomb to strike the city this month showed a "higher level of skill" than the previous three.

Fourth Austin bombing: in pictures Show all 8 1 /8 Fourth Austin bombing: in pictures Fourth Austin bombing: in pictures Investigators work at the scene of a bombing on Dawn Song Drive in the Travis Country neighborhood. Rex Fourth Austin bombing: in pictures Police tape marks off the neighborhood where the package bomb went off. It is the fourth similar bombing in three weeks in the Austin area. Getty Fourth Austin bombing: in pictures A police crime scene van arrives near the site of the explosion. Police warned nearby residents to remain indoors overnight as investigators looked for possible links to other deadly package bombings elsewhere in the city this month. AP Fourth Austin bombing: in pictures FBI and police investigate a bombing at the intersection of Republic of Texas and Mission Oaks boulevards. Rex Fourth Austin bombing: in pictures Officials work on the site of the explosion. AP Fourth Austin bombing: in pictures Members of Austin Police Department block off roads, Reuters Fourth Austin bombing: in pictures Police lines are seen blocking off part of Republic of Texas Boulevard. Reuters Fourth Austin bombing: in pictures Police maintain a cordon near the site of the explosion in southwest Austin. Reuters

Police Chief Brian Manley urged residents in a south-west neighbourhood of the city to be cautious of suspicious packages after Sunday's blast, triggered using a trip wire, injured two cyclists.

“The belief that we are now dealing with is someone who uses trip wires shows a higher level of sophistication, a higher level of skill,” Mr Manley said during a Monday morning press conference.

The carnage appears to be random, police said. It followed suggestions the package bombs were targeting minorities after two black men were killed by earlier devices.

Authorities, who have been unable to identify a suspect despite numerous tip offs, described an evolving strategy by the bomber.

They noted the bombs may now be triggered by twine, fishing line, or any number of trip wires, and that people should be careful around the city.