The FBI says the letters that were sent to President Barack Obama and to Sen. Roger Wicker of Mississippi are related -- and both were postmarked more than a week ago in Memphis, Tennessee.
The agency says the substance in both letters has tested positive in preliminary tests for ricin, a fatal poison. Both letters were intercepted at off-site mail facilities.
It's a poison that has sometimes been lumped in with other bioterrorism agents, because it comes from a relatively common plant -- the castor plant that makes castor oil -- and because it seems easy to make.
But in fact, ricin has created far more scares than victims. Experts say it's more of a targeted poison -- a tool for an assassin -- rather than something with which to attack a lot of people.
Both letters were intercepted before reaching their intended destinations.
Authorities in Michigan and Arizona also were on alert, because suspicious letters were sent to Michigan Sen. Carl Levin and U.S. Sen. Jeff Flake in Arizona.
Michigan authorities said they were still investigating the letter, and Arizona authorities said that nothing dangerous was found.
Security officials in Ohio were on alert because of the ricin scare and the recent bombings in Boston.
At the Ohio Statehouse, a scanning machine checks out every piece of mail as it arrives at the building.
Communications director Luke Stedke said an envelope or package may be checked if it looks out of the ordinary. This is actually the second stop that all statehouse mail makes before being delivered to lawmakers.
"It's scanned at the US post office, comes to our loading dock and it's scanned again; x-ray scanned. Then it goes to the senate mailroom," explained Stedke.
"I'll be honest. Opening the mail is one thing we normally don't think of as a security concern. But we definitely will now," said state Sen. Kevin Bacon.
Employees at Ohio Sen. Rob Portman's office in Columbus called police after they discovered a suspicious package sent to the building on Wednesday afternoon. After checking out the package, officials determined that it was not dangerous. A description of the package was not immediately available, and there were no evacuations during the investigation.
Portman's office released the following statement:
"Our team has communicated with staff about mail protocol and other security issues, and Sen. Portman's Ohio offices, like his DC office, are taking every precaution possible. Fortunately, no hazardous materials were delivered to Senator Portman's Columbus office."