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EARLIER THIS YEAR, threatening letters were sent to Baltimore’s City Hall and Clarence M. Mitchell Jr. Courthouse. At least two of the letters had bullets inside and at least one had a white powder, though it was later determined to be harmless. An investigation is ongoing as to the source of the mailings, but the letters caused a media flurry at the time as several of the recipients publicly asked how the threatening mailings got through the U.S. Postal Service (USPS) to the judges’ and lawmaker’s offices in the first place.

In 2002, a year after numerous anthrax attacks were made through the mail, biohazard detection systems (BDS) were installed in all of the nation’s USPS mail processing and distribution centers, says Peter Rendina, U.S. postal inspector. These BDS systems sample the air over the top of high-speed mail processing equipment; an alert should go off if a hazardous material is detected. The processing center will then be evacuated and shut down and postal inspectors will determine what the hazard is.

The mail items in the Baltimore incident would not have set off an alarm because they contained no hazardous substances; the powder included was harmless. A bullet alone is also not actually harmful. It is probably incorrect, therefore, to call the delivery of such letters a failure of the system.

Rendina points out that prior to delivery, mail is handled once more by a postal employee—the retail clerks and workers in postal distribution and processing centers. These workers have been trained to spot suspicious mail. Some of the tipoffs are lack of a return address, restrictive wording such as “personal,” and external evidence of powder, excessive tape, oil stains, or protruding wires. Rendina says the postal service fields about 38,000 reports of suspicious mailings each year; but these are usually false alarms—the problem being faulty packaging, rather than an actual threat or intentional hoax.

Some of the discretion as to whether a parcel or letter is further scrutinized arises from the weight of the package, according to Rendina. Any piece of mail weighing 13 ounces or more must be mailed in person with a retail clerk at a post office. If such a piece of mail is placed into a blue mail drop box, it will be returned to sender with a direction to take it to a retail clerk.

If there is no return address, the post office will contact the intended recipient and ask if the person was expecting a package at that time. If no information on the package can be garnered in that manner, it will be deemed suspicious and be further inspected and x-rayed, says Rendina.

The postal service cannot, however, go as far as x-raying or opening mail without clear evidence that it is warranted, explains Rendina. The postal inspector working on the Baltimore mailings case has increased screening of the mail to those facilities, taking it a “step further” than if there was nothing out of the ordinary going on. Any person or location experiencing threats can receive such temporary additional screening.

Redina adds that when mail is being sent to a government facility or other location, the onus would be on personnel at that government facility to implement additional screening measures. Postal service representatives are available to visit mail centers in companies and government buildings to describe extra screening that employees can do after the mail is delivered to the facility.

At least one of the targeted facilities has also taken action in light of the potential threat. While the Baltimore judiciary did not respond to calls for comment, Ryan O’Doherty, a spokesperson for the City of Baltimore, says that ongoing advanced security measures have been implemented at City Hall to address the security concerns. However, he would not discuss the specific measures.