A letter addressed to President Obama has tested positive in preliminary tests for the deadly poison ricin, the FBI confirmed Wednesday.
The letter is similar to one found Tuesday in a letter sent to a Mississippi senator, and came as several reports of suspicious letters and packages sent to lawmakers emerged. Also Wednesday, two mail screening facilities in Maryland were being investigated after tests showed the presence of ricin there.
The letter to Obama was intercepted Tuesday at one of those mail screening facilities. The Obama letter was "very similar" to a letter sent to Sen. Roger Wicker, R-Miss., that was found Tuesday, authorities said.
The letter to Wicker also has tested positive in the field for ricin, the FBI Field Office in Baltimore said.
The letters to both Obama and Wicker will now get further testing.
"There was a letter addressed to the President that at an off-site mail facility was noticed to contain a suspicious substance and tests were undertaken," said White House spokesman Jay Carney in a briefing Wednesday. "The FBI has the lead in that investigation, of course, and has said in its statement that they will be conducting further tests to determine what the nature of the substance is."
Authorities say they are aware of a person who they suspect could be linked to the Wicker mailing.
"The person that is a suspect writes a lot of letters to members," Sen. Claire McCaskill, D-Mo., said Tuesday as she emerged from a classified briefing.
Wednesday, several other reports of suspicious packages or envelopes emerged both in Washington and in lawmakers' offices back home. Sen. Carl Levin, D.-Mich., said a staff member at his Saginaw office received a suspicious letter Wednesday.
Sen. Jeff Flake's Phoenix office was evacuated briefly after reports of a suspicious package, though that package turned out to be innocuous.
The atrium of the Hart Senate Building was briefly evacuated as authorities examined a suspicious package there and two suspicious envelopes on another floor.
Authorities have revealed the most about the Wicker envelope, which contained a white granular substance and was intercepted about 11 a.m. Tuesday. It was intercepted at a Landover, Md., mail facility and did not reach the Capitol.
Both the Landover facility and another screening facility in Hyattsville, Md. were being investigated after tests indicated the presence of ricin.
Capitol Police spokesman Shennell S. Antrobus said police were notified that the mail facility had received "an envelope containing a white granular substance."
"The envelope was immediately quarantined by the facility's personnel and USCP HAZMAT responded to the scene," Antrobus said. "Preliminary tests indicate the substance found was ricin. The material is being forwarded to an accredited laboratory for further analysis."
One congressional official, speaking on condition of anonymity because the investigation wasn't concluded, said evidence of ricin appeared on two preliminary tests.
Terrance Gainer, the Senate's sergeant-at-arms, said the envelope bore a Memphis, Tenn., postmark but had no return address or suspicious markings.
Mail from a broad swath of northern Mississippi, including Tupelo, Oxford and DeSoto County, is processed and postmarked in nearby Memphis, according to a Postal Service map. The Memphis center also processes mail for residents of Western parts of Tennessee and eastern Arkansas.
Wicker's office has informed the senator's close associates of the tainted letters, a source close to Wicker told NBC News Tuesday. The office also is telling associates that no one at the Post Office was exposed to the substance.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," said a statement issued by Wicker. "I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. Gayle and I appreciate everyone's thoughts and prayers."
The letter's appearance "wouldn't raise suspicion," the Senate sergeant at arms said.
Mail already screened and processed to be delivered to Senate offices will be delivered, but mail service will be suspended Thursday and Friday.
House leadership said there has been no change in their mail service.
"It was caught at the screening facility,” McCaskill said. “That's why we have the offsite screening facility for mail. And the tests came back positive. And they are shutting down the post offices temporarily to make sure they get everything squared away and we are notifying our state offices what to look for."
Ricin is fatal if inhaled in a quantity as small as a few grains of table salt -- but it's still far less dangerous than the anthrax spores found in letters sent to the offices of two senators in 2001, officials said.
Stay with NBC Washington and News4 for more on this developing story.
Photo Credit: AFP/Getty Images