A midnight explosion at a FedEx facility near San Antonio was reportedly caused by a package bomb that was en route to Austin—where four such bombs within the last two weeks have killed two people and injured four others.

The mid-size package was on a conveyor belt in the sorting area of the facility at about 12:30 a.m. in the suburb of Schertz when it was detonated, police said in a press release early Tuesday.

One woman was treated at the scene for a headache, but there were no serious injuries reported.

Several outlets, including CBS Austin and The Washington Post, report the package was heading to an Austin address—and marked as coming from an Austin address—when it detonated. Both outlets noted that their information came from unnamed sources in two different agencies involved in the investigation at the facility.

According to CBS San Antonio, an FBI official said it was “more than possible” the early-morning bomb is related to the other local explosions. FBI Special Agent Michelle Lee told the Associated Press that “it would be silly for us not to admit that we suspect it’s related.”

Tuesday’s package reportedly contained nails and other metal shrapnel, like the four previous explosions that have rattled the Texas capital. Agents were searching other packages at the facility to ensure they contained no other explosives, and at least 75 FedEx employees were being questioned Tuesday morning by officials from the FBI, ATF, and Homeland Security agencies, several outlets reported.

All of the other bombs this month were hand-delivered to the scenes, but officials told The Daily Beast on Monday that the attacker—or attackers—have been evolving their modus operandi. A bomb that wounded two men on Sunday evening in southwest Austin was the first this month to be detonated with a tripwire, and the attacks have been happening with increased frequency. Ten days separated the first bombing from the next two, but Sunday’s bomb went off just six days later.

“That’s a level of diabolical sophistication to be able to move that quickly,” former FBI profiler Clint Van Zandt told The Daily Beast.

“It’s a race against the clock right now,” he said. “This person—or persons—has shown us they’re fully capable of killing.”

Between federal and local authorities, officials estimate that at least 1,000 local law-enforcement agents are actively searching for the bomber—or bombers.

The first three package bombs were all detonated on the east side of the city, killing 39-year-old Anthony Stephan House and 17-year-old Draylen Mason and wounding a 75-year-old Latina resident. Each of those packages were left on the victims’ front porches. Mason’s mother was also seriously wounded in one of the attacks.

In Sunday’s attack, two men were seriously injured at about 8:30 p.m. on a residential street in southwest Austin after they detonated a tripwire connected to a “suspicious package,” police said.

Austin Police Chief Brian Manley on Monday said officials believe they are dealing with a “serial bomber” terrorizing the Texas capital and pleaded with the suspect to come forward.

“These events in Austin have garnered worldwide attention, and we assure you that we are listening,” he said. “We want to understand what brought you to this point, and we want to listen to you.”