This week, apparent explosive devices targeted the mailboxes of former President Barack Obama, former presidential candidate Hillary Clinton, philanthropist George Soros, and cable network CNN. Additional reports of suspicious packages have continued to emerge Wednesday, and the situation is still developing. What all of the attempts have in common—aside from prominent liberals as the intended recipients—is that they have, so far, been intercepted before they could cause any damage.

In the Soros case, authorities reportedly believe that the bomb was hand-delivered, rather than going through a parcel service; a caretaker discovered the package and contacted authorities, who detonated it safely. The other devices, though, appear to have been caught in advance of their final destination by standard screening processes.

“The US Secret Service has intercepted two suspicious packages addressed to Secret Service protectees,” the agency said in a statement on Wednesday, referring to the items addressed to Clinton and Obama. “The packages were immediately identified during routine mail screening procedures as potential explosive devices and were appropriately handled as such. The protectees did not receive the packages, nor were they at risk of receiving them.”

A third suspicious package, meanwhile, was found in the mailroom of the Time Warner Center, where CNN is headquartered. In a press conference Wednesday, NYPD Commissioner James O'Neill said the package contained an explosive device, and that additionally there was an envelope with white powder, which is being investigated. Further reports have indicated that Representative Maxine Waters of California, among others, had also been sent suspicious packages. New York Governor Andrew Cuomo said that "a device" had been sent to his office as well, yet the NYPD confirmed to WIRED that the package was "deemed to be non-suspicious."1

While the Secret Service and CNN have not commented further, the mail screening that sniffs out potential explosives—not to mention biochemical threats—follows fairly standard protocol.

"Depending on device construction, some people have a signature." Michael O'Neil, MSA Security

First, there are some common warning signs, according to guidelines from the US Postal Inspection Service, which says it has investigated “an average of 16 mail bombs over the last few years.” Parcels that have excessive postage, for instance, suggest that the sender has sought to avoid direct contact with postal employees at a local branch. The USPIS suggests, too, that mail bombs tend to specify “personal” or “private” delivery—often in distorted handwriting or homemade labels, in an attempt to avoid creating evidence. They also may just look weird, with an unusual shape or even protruding wires or oil stains. NBC News posted an image of the purported package sent to CNN, which appears to check many of those boxes.

When contacted by WIRED, the USPIS declined to detail its investigative procedures and operational protocols, citing security concerns. But it does have a system in place. “The Postal Inspection Service has organized response teams nationwide for investigating suspicious parcels through our Dangerous Mail Investigations Program,” the agency said in a statement. “DMI Inspectors are trained to recognize the common characteristics of suspicious mail and are highly proficient in the use of state of the art equipment to include portable x-ray machines.”

Even packages with no exterior giveaways can be fairly straightforward to sniff out, according to Michael O’Neil, former commanding officer of the NYPD counterterrorism division—especially for private screening services, like a company or the Secret Service.

“What’s common in a lot of corporate structures, here in New York City especially, there’s some type of mail service that does some type of screening. Primarily that’s an x-ray machine; they’ll put a package through before it gets up to its intended addressee,” says O’Neil, who is currently the chairman of MSA Security, a private threat protection company whose offerings include explosive screening services. If an x-ray operator sees something suspicious, for instance, they can send the image to MSA Security, whose bomb technicians will help evaluate the threat.