Law enforcement authorities said Wednesday morning that they have intercepted a second letter containing a substance testing positive for the poison ricin--this one addressed to the White House.

News of the tainted letter dated April 16 to President Obama comes one day after officials said they had intercepted a letter testing positive for ricin intended for Sen. Roger Wicker, R-Miss., and bearing a Memphis, Tenn., postmark.

The FBI says there is no indication of any connection to the letters with the Monday bomb attack in Boston.

But the investigation into these letters remains ongoing, and more letters may still be received. In fact, Sen. Carl Levin, D-Mich., said in a statement Wednesday that his Saginaw regional office had received a suspicious-looking letter. It was not opened, he said, and officials are still determining whether it is tainted in some way. And in Arizona, there are reports of suspicious letters sent to the Phoenix offices of Republican Sens. John McCain and Jeff Flake.

According to the FBI and the Secret Service, the envelope addressed to the White House was received at a remote White House mail screening facility. This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery. As with the similar screening center used for congressional mail, this facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.

The envelope, addressed to the President was immediately quarantined by U.S. Secret Service personnel, and a coordinated investigation with the FBI was initiated. It was then that filters at a second government mail screening facility preliminarily tested positive for ricin this morning. Other mail from that facility is being tested.

Meanwhile, a U.S. Capitol Police spokesman said their officers on Wednesday morning set up two command posts inside the Russell Senate Office Building and in the atrium of the Hart Senate Office Building -- and that an individual has been detained – after the discovery of two “suspicious” envelopes found in the buildings.

All staff and others were being told to avoid the areas as the investigations continue. Officials were searching Sen. Richard Shelby’s office in Russell, because a suspicious package was delivered there.