Amid the letters to Obama and Wicker, other scares were reported Wednesday.
A sensor was activated at a third federal government mail-sorting facility Wednesday, sources said. Investigators were culling through the mail to see whether they could identify a suspicious letter.
The facility, they said, sorts mail for the Department of Justice. The sensor was undergoing additional testing at a lab. A source cautioned that the sensor activation might involve a false positive and not be related to the other letters.
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 was working closely with the U.S. Capitol Police and the FBI in the 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 FBI 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 said any time 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 were in the process of being conducted and generally take from 24 to 48 hours.
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, Neb., 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' Michael S. James and Sunlen Miller contributed to this story.