Boston Mystery Deepens as New One Unfolds in D.C.

A frustrating day of no leads or suspects emerging from the FBI's quest to track down the Boston Marathon bomber took a strange turn Tuesday in Washington as a letter to a Republican senator tested positive for ricin.

Mail making its way to the Hill has been screened off-site for toxic substances ever since the 2001 anthrax letters, and it was here that the letter to Sen. Roger Wicker (R-Miss.) was intercepted.

"Earlier today the United States Capitol Police was notified by the Senate mail handling facility that it 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. Preliminary tests indicate the substance found was Ricin," said Public Information Officer Shennell S. Antrobus in a statement.

"The material is being forwarded to an accredited laboratory for further analysis. The USCP is partnering with the Federal Bureau of Investigation to investigate this incident. This is an ongoing investigation." Lab tests in Maryland and field tests were positive.

Wicker, who has been placed under protection, simply added it was an ongoing investigation and said he and his wife "appreciate everyone's thoughts and prayers."

"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," the senator added.

Senators were told about the incident at an early evening briefing led by Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller that was expected to focus on information about the Boston terrorist attack. Sen. Claire McCaskill (D-Mo.) wrongly told reporters that a suspect was in custody for the ricin letter, which had a Mempis postmark and no return address.

For apparently having so little information about the bombing, the administration tried to convey the message that Americans didn't need to fear a follow-up attack -- even as a nervous White House briefly evacuated today because of a suspicious package.

"What I've indicated to you is what we now know. We know it was bombs that were set off. We know that, obviously, they did some severe damage. We do not know who did them. We do not know whether this was an act of an organization or an individual or individuals. We don't have a sense of motive yet," President Obama said during a morning statement. The daily press briefing with spokesman Jay Carney was canceled, and Obama didn't take questions from reporters after his remarks.

In her own statement, Napolitano said she was in contact withe Boston officials and the state's congressional delegation to stress "the Department of Homeland Security’s support to the FBI-led investigation and the Administration’s commitment to bringing whoever committed this act of terror to justice."

"While there is no current indication to suggest that the events in Boston are indicative of a broader plot, out of an abundance of caution, DHS continues to keep in place enhanced security measures at transportation hubs, utilizing measures both seen and unseen," Napolitano continued. "We continue to urge the American public to remain vigilant and immediately report any signs of suspicious activity to local law enforcement officials.”