Panic unfolded yesterday after a series of pipe bombs were mailed to CNN and prominent critics of Trump. As of Thursday morning, at least 10 bombs have been discovered, including packages sent to George Soros, Robert De Niro, and Joe Biden.

While the CNN delivery made its way to parent company Time Warner in New York, triggering a massive law enforcement response, packages meant for Hillary Clinton and Barack Obama were never delivered. Moreover, the packages have been obtained intact, giving investigators a wealth of potential clues. Law enforcement will now sift through all of the evidence in the hunt for the culprit.

Crucially, the intercepted packages were obtained by authorities before detonation. In a statement on the incident, the Secret Service said that mail screening had caught the packages.

“The packages were immediately identified.”

“The packages were immediately identified during routine mail screening procedures as potential explosive devices and were appropriately handled as such,” the agency said in the statement. “Both packages were intercepted prior to being delivered to their intended location. The protectees did not receive the packages nor were they at risk of receiving them.”

The exact logistics of today’s interception aren’t perfectly clear, and the Secret Service has yet to release a more detailed statement. But one of the agency’s key duties is the collection and examination of mail from protected persons. Historically, that’s meant the White House, but the service also watches over former presidents and major party candidates.

Since 1996, the agency has operated remote delivery sites for mail sent to the president, and similar screenings are routine for protected persons outside of the White House. That means checking “anything that comes in or comes out,” says Larry Johnson, CEO of CyberSponse and former head of criminal investigations at the Secret Service and a supervisor of the Presidential Protective Division at the White House. That includes screening people as well as offsite screenings of mail, Johnson says, where tools like X-rays, biological sensors, and trace detection can be used. “There’s a lot of technology out there,” he says. The explosive devices seen so far have been described as “crude,” suggesting it might not take more than a simple examination to determine their contents.

Secret service employs “state of the art X-raying screening”

While the “routine” systems that caught the bombs in this case haven’t been outlined, the United States Postal Service also generally monitors the public’s mail for signs of explosives, as the mail system has long been used for terror. The US Postal Inspection Service acts as the agency’s law enforcement arm. In a statement, the Inspection Service said it was unable to detail the chain of events that led to the package being detected, but it said it is looking into the incidents. “Although the U.S. Postal Inspection Service is working closely with our law enforcement partners on this investigation, we cannot speak to specifics of the investigation.”

For now, the packages are being sent to an FBI lab in Virginia, which is the first step in the forensic search for their sender. The packages have already left behind clues that investigators can now examine. Trace evidence could include lingering DNA or fibers found on the packages. Johnson says another valuable tool could be the Secret Service’s running database of people who have made threats in the past. Does something on the package match the M.O. of a previous threat, or is there some other way to make an ID?

Clues left behind on the packages will also play a crucial role. Investigators will begin disassembling the packages in an attempt to trace them back to their sender. “What you have is materials — whether it’s paper, whether it’s stamps, not to even mention the device, which you have to purchase the materials [for],” Johnson says. Any of those could be traced back to their buyer and then to the sender of the packages.

By this time, investigators are likely looking at several angles. “I would assume there’s some very solid leads,” Johnson says.