A letter addressed to President Obama that field-tested positive for the poison ricin was received at the remote White House mail screening facility Tuesday, according to law enforcement officials.
Law enforcement officials emphasized that these tests are preliminary and are often unreliable. They said the final word would come from a complete lab test that should be completed in the next 24 to 48 hours.
A false positive for ricin occurs at least once each year, a homeland security official told ABC News.
The facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.
Law enforcement officials emphasized that these tests are preliminary and are often unreliable. They said the final word would come from a complete lab test that should be completed in the next 24 to 48 hours.
A false positive for ricin occurs at least once each year, a homeland security official told ABC News.
The facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.