A "suspicious substance" was found in a letter sent to US President Barack Obama, the US Secret Service said on Wednesday.
"On 4-16-13, a letter addressed to the (US) President containing a suspicious substance was received at the remote White House mail screening facility. This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," a spokesman of the US Secret Service said.
The Secret Service White House mail screening facility is a remote facility, not located near the White House complex, that all White House mail goes through.
"The Secret Service is working closely with the US Capitol Police and the FBI in this investigation," the Secret Service said.
The White House has been put under heightened security alert after Monday's bomb blast in Boston that killed three persons and injured more than 170. The security perimeter around the White House has been increased.
A day earlier, an envelope addressed to a senior Republican Senator was tested positive for ricin, a poisonous substance, which added to the heightened concerns in the United States.
Senate Majority Leader Harry Reid told reporters at the Capitol Hill that the letter was addressed to Senator Roger Wicker from Mississippi, which has tested positive for ricin. His security has been increased.
According to the Centers for Disease Control and Prevention, Ricin is a poison found in castor beans, which can be manufactured from castor bean waste materials.
After the 2001 anthrax scar, all the mails sent to the lawmakers are screened at an off-site facility. The letter was tested positive at this off-site facility.
"There was a letter sent to a member that had gone through our processing facility - not on-site here, but in the area - and it was identified as containing Ricin, which is a dangerous substance," Senator Mary Landrieu told reporters after a briefing for Senators on the investigation of the Boston bombings.
"The police are in full investigation. They don't think that anyone in the Capitol complex is in any danger. They'll go through regular decontamination procedures and they'll notify all the appropriate offices," Landrieu said.