A package destined for Austin, Texas blew up in a FedEx facility just after midnight in Schertz, just down Interstate 35 from the state capital where four other bombings have taken place over the last three weeks. The bomb contained nails or similar shrapnel devices, but the only injury reported thus far resulted from the sound of the explosion. The FBI and ATF have responded, not just to look for evidence from the explosion but also to determine whether any more unexploded bombs are in the facility:

They may be there for a while, and don’t count on overnight delivery to the area:

According to police, who spoke with San Antonio ABC affiliate KSAT, a medium-sized box exploded at the facility.

One worker possibly suffered minor injuries in the explosion. They were not taken to the hospital, the official said. There were 75 people in the building at the time of the explosion.

Police said they “hope” this is an isolated incident and do not yet know if it is connected to the Austin bombings.

A law enforcement source tells ABC News that it is going to take a considerable amount of time before any post-blast investigation can occur. The site is complicated and potentially dangerous to render safe, with other FedEx packages of unknown contents all over. Authorities will have to move slowly and methodically to process the complex scene.

NPR reports that it could have been worse — much worse. The device apparently detonated prematurely when few people were around:

“[T]he package was moving from an elevated conveyor belt to a lower section when it exploded,” the television station reports.

The package “contained shrapnel consisting of nails and pieces of metal, sources said,” according to the CBS affiliate, which said the Schertz facility has 75 employees.

If it’s the same bomber as behind the Austin series, then the perp has gotten more sophisticated — and perhaps gambling more, too. The initial attacks were hand-delivered packages that avoided any potential tracking or handling. The most recent attack didn’t use packages at all, but rather a trip-wire design that was unfortunately effective:

The latest blast, which injured two men walking through a residential area Sunday night, marked an escalation in both the tactics and skills displayed by the bomber or bombers, police said. While the three previous devices were hidden in packages delivered to homes in residential neighborhoods, the fourth device — anchored to a for-sale sign — was left on the side of the road and was rigged with a tripwire, showing “a higher level of sophistication, a higher level of skill,” said Brian Manley, the interim Austin police chief.

“What we have seen now is a significant change from what appeared to be three very-targeted attacks to what was, last night, an attack that would have hit a random victim that happened to walk by,” Manley said at a news briefing Monday. “So we’ve definitely seen a change in the method.”

Manley said the fourth device had “similarities” to the three bombs that detonated in the Texas capital in the past three weeks, killing two people and injuring two others, one seriously. The explosion Sunday night plunged the city further into a frightening uncertainty that has left residents on edge. Authorities have seemed vexed, completely at a loss to explain who could be setting off the devices — or why — and asking the unknown attacker to communicate with them. They also have offered rewards of up to $115,000 for information in the case.

The first three explosives all hit eastern Austin, affecting areas where black and Hispanic residents live, prompting some to question whether the initial blast on March 2 would have prompted more urgency had it gone off in a more affluent, predominantly white neighborhood — like where the attack Sunday night happened, injuring two white men.

If the FedEx explosion came from a device built by the same bomber, it seems that targeting demographics isn’t the point. It’s still possible that the FedEx bomb came from someone else, but that would mean that two different domestic terrorists are operating in the Austin-San Antonio region at the same time using similar attack methods. That seems a little too coincidental to believe, but it’s not entirely out of the question, either.

The big question everyone wonders is why. What is the motive, and what does the bomber want? Law enforcement took the extraordinary step of asking the bomber to contact them in order to set up a “dialogue.” At some point, the perp or perps will want their motive known, in manifestos or declarations of some kind. Meanwhile, the residents of Austin now not only have to worry about unexpected packages without legit shipping labels but even those that do have them, plus have to worry about walking down their own sidewalks. Rarely has if you see something say something seemed so necessary.

Update: Police are now responding to another suspicious package call at a FedEx facility in Austin:

We’ll keep an eye on this to see what develops.

Update: I believe this was in MSNBC’s report, but it’s worth noting separately:

Is that a standard response to a bomb threat, or something more specific to this incident?

Update: Looks like there was more than one in Schertz, too:

It looks like the serial bomber is trying to flood the zone.