Five "potential explosive devices" sent to Hillary Clinton, former President Obama, billionaire George Soros, ex-Attorney General Eric Holder and CNN at NYC's Time Warner Center are thought to be linked, law enforcement sources say -- and officials are looking into whether one addressed to California Rep. Maxine Waters in Washington, D.C., has a similar signature.
The packages that were addressed to Soros, Clinton, Obama, Brennan (through CNN), and Holder have the same characteristics, three senior law enforcement officials with direct knowledge of the matter tell NBC News.
The targets of the packages are leading some to speculate on a motive, but experts say it's far too early for that.
"There's an old saying with investigations, and that's 'assumptions are your enemy,'" said Michael Clark, a retired FBI special agent who now teaches criminal justice at the University of New Haven.
Law enforcement will start with a clean slate, focusing on the physical evidence in front of them.
"They'll take a very close look at all the materials. The person who did this has got to be asking themselves, 'Did I make a mistake? Did I touch the envelope? Did I lick a stamp? Did I lick the envelope closed?'" said Clark.
Clark says the suspicious packages will be looked over by three of the best labs: the Secret Service lab, the FBI lab, and the ATF lab.
"These bombers leave a signature every time they create one of these bombs. They'll check it against all the different databases they have for these bombing signatures," said Clark.
Clark says investigators will keep their best leads close to the vest and that the key will be forensics.
"It's going to come to forensics and shoe leather with the investigators hitting the street with leads. They're going to work around the clock. This is a top priority investigation. Nothing will be more important than this investigation," said Clark.
Quinnipiac University Political Science Associate Professor Fodei Batty says a lot of people are certainly going to say the mail bombings were due to the partisan divide, but that it's important to see what comes of the investigation.
"We can't really rush to judgment. We have to wait and see," said Batty.
He adds that whatever the motive ends up being, it's certainly bringing attention to the tense political polarization in the country, and that everyone should take a step back and evaluate the discourse.
"I have never seen American politics this divided," said Batty. "There should be more civility injected into public discourse."