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.”

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.