A suspicious letter mailed to the White House and intercepted this week was similar to two threatening, poison-laced letters sent to New York Mayor Michael Bloomberg, the US Secret Service said.
Letters sent to Mr Bloomberg and his gun control group contained material believed to be the deadly poison ricin and contained a reference to gun control.
A Secret Service official said the letter sent to the White House was similar but did not elaborate.
The White House mail screening facility turned it over to the FBI's Joint Terrorism Task Force.
It was not immediately clear whether the letter sent to Mr Obama contained ricin.
The two Bloomberg letters, opened last Friday in New York and Sunday in Washington, contained an oily pinkish-orange substance.
New York Police Department Commissioner Raymond Kelly said the same machine or computer had produced the two letters to Mr Bloomberg and the similar one to Mr Obama and that they may be identical.
The FBI said in a statement that field tests on the letters were consistent with the presence of a biological agent.
The body of the letter mailed to New York was addressed to "you" and referenced the gun control debate.
Commissioner Kelly said the unsigned letter says, in so many words: "Anyone who comes for my guns will be shot in the face."
The people who initially came into contact with the letters showed no symptoms of exposure to the poison, but three officers who later examined the New York letter experienced minor symptoms that have since abated, police said.
According to the federal Centre for Disease Control and Prevention, ricin is a poison found naturally in castor beans.
Symptoms can include difficulty breathing, vomiting and redness on the skin depending on how the affected person comes into contact with the poison.
