The department also said that besides Obama and Wicker, a third letter was sent to a justice official in Mississippi. The letters addressed to Obama and Wicker were intercepted at a government mail-screening facility after preliminary tests showed the presence of deadly poison ricin in them.
The envelopes were later sent for additional tests for confirmation, sources in the FBI said. The results were expected on Thursday, they added.
The letters had the following message: "To see a wrong and not expose it, is to become a silent partner to its continuance." They were signed "I am KC and I approve this message", one source said.
Reports also said that suspicious packages and envelopes reached the two Senate buildings on Wednesday morning. Police evacuated the first floor of the Hart Senate Office Building and interrogated a man who had a backpack containing sealed envelopes. He was not taken into custody though.
Meawnhile, reports came that some other officials also received envelopes with a substance substance and a letter similar to those sent to Obama and Wicker.
The FBI said there is no indication of a link between the letters and Monday's bombings at Boston that killed three and injured several others. However, the discovery of the letters heightened security worries at a time when the Congress is considering a challenging legislation to toughen gun laws and reform the immigration procedures.