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. A company spokeswoman refused to say if that second package might have been linked to the one reported at the distribution center near the airport.

The Schertz blast came less than two days after a bombing wounded two men Sunday night in a quiet Austin neighborhood about 3 miles (5 kilometers) 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.

A criminologist at the University of Alabama said if a single perpetrator is behind the blasts, changing the means of delivery increases the bomber's chance of getting caught.

"I think it would suggest that the bomber is trying to stay unpredictable," Adam Lankford said. "But it also increases the likelihood that he would make a mistake."Authorities have not identified the two men who were hurt Sunday, saying only that they are in their 20s and white. But William Grote told The Associated Press that his grandson was one of them and that he had what appeared to be nails embedded in his knees.

On the night of the fourth bombing, one of the victims was riding a bike in the street and the other was on a sidewalk when they hit the tripwire.

"It was so dark they couldn't tell, and they tripped," Grote said.

In Washington, US President Donald Trump said the assailant behind the bombing is "very sick."During an Oval Office meeting Tuesday with Saudi Arabia's Crown Prince Mohammed bin Salman, the president said, "This is obviously a very sick individual or individuals," and authorities are "working to get to the bottom of it."Despite bombing tactics that have now shifted, investigators have repeated prior warnings about not touching unexpected packages and urged people to be wary of any stray object left in public. Austin police say they have now responded to more than 1,200 reports of suspicious packages in a little more than a week — without finding anything dangerous.

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

"We are clearly dealing with what we believe to be a serial bomber," Manley says.

AP