Updated 8:34 PM ET:late Wednesday, CBS News has learned.
(CBS News) A letter sent to the president tested positive for a powerful poison called ricin, and another envelope to a U.S. senator was intercepted Wednesday.
The letters set off alarms at three complexes in Maryland, where the U.S. Post Office intercepts all mail addressed to Federal officials. Outside those facilities Wednesday, hazmat teams continued to check for ricin contamination.
According to the FBI, there were three letters containing a white granular substance: two sent to Mississippi Sen. Roger Wicker and one to President Obama. On Wednesday evening, the Department of Justice announced the arrest of Paul Kevin Curtis from Corinth, Miss., in connection with the mailings.
Ricin is a poison that originates in castor beans. It's easy to make and can be lethal when small doses are inhaled.
White House spokesman Jay Carney said the president was never in danger because the post office screening system worked.
"There is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now," Carney said.
The letter arrived one day after the Boston bombing, but the FBI says, for now, "There is no indication of a connection to the attack in Boston." The letters were mailed in Memphis, Tenn., last week on April 8, and seemed to have come from one individual, who wrote:
"No one wanted to listen to me before. This must stop. To see a wrong and not expose it is to become a silent partner to its continuance. I am KC and approve this message."
The FBI believes Curtis has sent other letters with similar content to other members of Congress in the past, and that pattern helped lead to his arrest.