A letter addressed to President Obama that field-tested positive for the poison ricin was received at the remote White House mail screening facility Tuesday, according to law enforcement officials.
The facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.
The separate Senate mail-handling facility also Tuesday received a suspicious letter potentially laced with ricin addressed to Sen. Roger Wicker of Mississippi, postmarked from Memphis.
Testing on the first letter, addressed to Republican Wicker, is incomplete but expected to be finished this afternoon.
The Secret Service's White House mail-screening facility is a remote facility, not located near the White House complex, through which all White House mail goes.
The Secret Service is working closely with the U.S. Capitol Police and the FBI in this investigation.
"The investigation into these letters remains ongoing, and more letters may still be received," the FBI said in a statement this morning. "There is no indication of a connection to the attack in Boston."
The Bureau added: "It is important to note that operations at the White House have not been affected as a result of the investigation.
"Additionally, filters at a second government mail screening facility preliminarily tested positive for ricin this morning. Mail from that facility is being tested."
FBI sources say anytime suspicious powder is located in a mail facility, field tests are conducted. The field and other preliminary tests in this instance produced inconsistent results. The material has been sent to an accredited laboratory for further analysis.
Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests are in the process of being conducted and generally take from 24 to 48 hours.
Field tests are often unreliable, and a false positive for ricin occurs at least once each year, a homeland security official told ABC News.
The Centers for Disease Control defines ricin as a poison that comes from castor beans and can be found in a powder, a mist, a pellet or dissolved in water.
"In the 1940s, the U.S. military experimented with using ricin as a possible warfare agent," the CDC writes. "In some reports ricin has possibly been used as a warfare agent in the 1980s in Iraq and more recently by terrorist organizations."
Meanwhile, Senate offices were on partial lockdown today after the discovery of suspicious packages. The police investigation centered on the offices of Sens. Richard Shelby, R-Ala., in the Russell Senate Building, and Joe Manchin, D-W.Va, in the Hart Senate Office Building.
The lockdown was unrelated to the Wicker letter.
Also, the Saginaw, Mich., office of Sen. Carl Levin, D-Mich., received a suspicious-looking letter this morning. The office did not know whether it was a threat.
"The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating. We do not know yet if the mail presented a threat," Levin said in a paper statement.
"I'm grateful for my staff's quick response and for government personnel at all levels who are responding."
Several senators have reported suspicious packages delivered to their district offices, with no reports of any credible threats.
But underscoring the jitters among Senate D.C. and district offices, Sen. Deb Fischer's, R-Neb., Lincoln office contacted police when staff found a suspicious package outside this morning.
It turned out to be a used-car part left in a bag on top of a lawn chair.
ABC News' Sunlen Miller contributed to this story.