By Jon Herskovitz and Jim Forsyth

AUSTIN/SCHERTZ, Texas (Reuters) - A package bomb blew up at a FedEx distribution center near San Antonio on Tuesday, the fifth in a series of attacks that have rocked Texas this month and left baffled investigators searching for what they suspect is a serial bomber.

The package filled with nails and metal shrapnel was mailed from Austin to another address in Austin and passed through a sorting center in Schertz, about 65 miles (105 km) away, when it exploded on a conveyer belt, knocking a female employee off her feet, officials said.

It was the fifth of a series in explosions in Texas in the past 18 days that have killed two people, injured others, and left hundreds of federal and local investigators scrambling to find the perpetrator and a motive.

"We do believe that these incidents are all related. That is because of the specific contents of these devices," interim Austin Police Chief Brian Manley told members of the Austin City Council, the Austin American-Statesman reported.

A further package sent by the same person was discovered and turned over to law enforcement, FedEx Corp said in a statement on Tuesday.

Fedex did not give further details on the second package but a FedEx employee with knowledge of the incident said another box had been tracked to a south Austin facility. The facility was evacuated, said the employee, who asked for anonymity because he was not authorized to discuss the matter.

Speaking through the media, officials have appealed to the bomber to reveal the motives for the attacks. They have also asked the public for any tips, offering a $115,000 reward for information leading to the arrest and conviction of the culprit.

"Somebody has to know something," said Federal Bureau of Investigation spokeswoman Christina Garza. "The person behind these explosives, please, we want to know why."

"This is obviously a very, very sick individual, or maybe individuals," President Donald Trump told reporters. "These are sick people, and we will get to the bottom of it."

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The four previous explosions - which took place between March 2 and March 18 - killed two people and injured four others, unnerving residents of Austin, the state capital of some 1 million people.

The first three devices were parcel bombs dropped off in front of homes in different Austin neighborhoods. The fourth went off on Sunday night on the west side of the city and was described by police as a more sophisticated device detonated through a trip wire.

MOTIVE?

Investigators were trying to work out a motive for the bombings and identity of the bomber or bombers, a U.S. security official and a law enforcement official told Reuters.

The FBI was investigating the FedEx package explosion on the assumption of a connection to the Austin bombings, the law enforcement official said. Both sources declined to be identified.

Security experts said the recovery of the second Fedex package and the attacker's use of a commercial parcel service could give investigators important clues.

Police have not given any details about the address on the second parcel or whether it contained any kind of device, saying only that it was a suspicious package.

But if it did contain an unexploded device, that "will be of great forensic value, then you will be able to tell unequivocally the construction techniques that are used," said Danny Defenbaugh, a Texas-based security consultant who worked for the FBI for 33 years.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) were among those working with local officials in Austin, Schertz and San Antonio.

"We have agents from across the country. We have our national response team here. We have explosive detection canines here. We have intel research specialists," Frank Ortega, acting assistant special agent in charge of the San Antonio ATF office, told reporters. "We've been working around the clock."

(Additional reporting by Brendan O'Brien in Milwaukee, Mark Hosenball and Lisa Lambert in Washington, Andrew Hay and Eric Johnson; Writing by Daniel Trotta and Rosalba O'Brien; Editing by Jeffrey Benkoe and Tom Brown)