(CNN) – Bombs and chemicals sent through the mail are rare but terrifying.
According to the U.S. Postal Inspection Service, more than 170 billion pieces of mail every year are processed, and only a tiny fraction of those – 16 over the last few years – were investigated as mail bombs.
That, according to the agency, works out to be an average of less than one in 10 billion.
So, how does law enforcement track down those who send suspicious packages like the several discovered Wednesday?
Former FBI and U.S. Marshals agents told CNN the manhunt for the person who delivered pipe bombs to CNN and others is now one of the most intense searches in recent memory.
"This is number-one. This is all hands on deck. Not just in New York. Not just here in D.C. But across the country," said Art Roderick, the former assistant director for the U.S. Marshals.
Experts said the perpetrator left important clues.
The packages didn’t explode, which means fingerprints and DNA could be retrievable.
A law enforcement official tells CNN all the devices appeared to be constructed similarly: pipe bombs, at least one containing projectiles.
At least two of the packages, addressed to Democratic donor George Soros and to former CIA director John Brennan at CNN’s Time Warner Center, are believed to have been delivered by hand.
The one sent to CNN was delivered by a courier, so surveillance footage can be used to track that.
Roderick said it’s possible that a network of people could be behind the crimes.
"I mean, they're not eliminating anything at this point in time,” he said. “But conspiracies are very difficult to keep under wraps. And generally when you have more people, somebody's going to say something or somebody's going to know something."
Experts said this is a multi-track investigation.
One crucial component is a profile of the subject.
Mary Ellen O’Toole profiled Ted Kaczynski, the Unabomber, for the FBI. She said investigators are looking closely at the connection between Wednesday’s targets.
Brennan – who the bomber mistakenly thought worked for CNN – and the other targets are prominent people in politics, who’ve all been harshly criticized by President Donald Trump or his right-wing allies.
Law enforcement will also look for specific characteristics in a possible suspect.
“[The suspect could be] someone that is very angry about the politics in the country and specifically against these individuals,” O’Toole said. "The reason that that’s important is because there may have been some effort prior to mailing and placing these bombs, to contact these people, either through a letter or through posting something on Facebook.”
O’Toole said the suspect could plan on mailing more bombs, could have active devices or other weapons on them when law enforcement approaches, and likely has a desperate outlook.
"This is someone who now cannot go back,” O’Toole said. “He cannot change things and blend back into a common, ordinary life. So, he has thrown all caution to the wind. So he has nothing to lose, to hurt or kill law enforcement as they approach him."
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