A suspicious letter sent to US President Barack Obama is "similar" to two poisoned letters mailed to New York City Mayor Michael Bloomberg this week, US officials have said.
The US Secret Service said the letter to Mr Obama was intercepted and was being tested by FBI investigators.
The letters to Mr Bloomberg referred to his support for stricter gun control.
One was delivered to the Washington DC office of Mr Bloomberg's gun control group Mayors Against Illegal Guns.
The other was addressed to the New York City mayor's office but intercepted at a mail sorting facility.
In a separate case, a Mississippi man is charged with sending ricin-laced letters to Mr Obama, a judge and a Mississippi senator.
Another man has been arrested in Washington state in connection with letters sent to a judge. On Thursday, the FBI confirmed another letter containing ricin was addressed to Mr Obama from the same location on the same day those messages were sent.
'Shoot in face'
Police have said preliminary testing of the letters sent to Mr Bloomberg indicated the presence of ricin, a poison extracted from castor beans.
One thousand times more toxic than cyanide, it can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.
Law enforcement officials have told US media that all three letters were marked as having been sorted in a facility in Shreveport, Louisiana.
A Louisiana State Police spokeswoman said the Shreveport postal centre handled mail from Louisiana, Texas and Arkansas, so the letter could have come from any of those states.
New York City Police Commissioner Ray Kelly told reporters on Thursday he believed that the letter addressed to Mr Obama was identical to those sent to Mr Bloomberg.
According to Mr Kelly, the letters contained a threat to "shoot in the face" anyone who came for the sender's guns.
Civilian personnel who came into contact with the letters experienced no symptoms. Minor symptoms in emergency workers who handled the letter at the sorting facility for the mayor's office "have since abated", the New York Police Department said in a statement.
Mayor Bloomberg is one of the most prominent proponents of stricter gun control laws in the US.
The firearms debate divides Americans and has leapt to the top of the political agenda since 26 people were killed in a school shooting in Connecticut in December.