Federal officials told NBC News follow-up laboratory tests were "inconclusive," and it wasn't clear whether the material in the letters was harmful.
Curtis of Tupelo, Miss., "may have stumbled onto something" but may not have successfully produced a fatally toxic form of ricin.
Ricin is a potentially deadly poison found in ground castor beans that disrupts the absorption of proteins on cells. There is no antidote.
The letter arrived at the White House mail facility Tuesday, the same day it was announced a letter addressed to Wicker had tested positive for ricin. The Wall Street Journal said officials indicated the letter to Obama initially tested positive for ricin.
There was a scare on Capitol Hill Wednesday morning when authorities evacuated parts of the U.S. Capitol complex because of suspicious packages found on the first and third floors of the Hart Senate Office Building and a suspicious envelope at the office of Sen. Richard Shelby, R-Ala, in the Russell Senate Office Building.
The suspicious packages were removed and Capitol Police reopened both buildings at 12:40 p.m., The Hill said.
A public address announcement said tests on the packages in the Hart Senate Office Building were negative.
Security had been increased in House and Senate office buildings since Tuesday.
Capitol Hill police reportedly were questioning a man with a backpack in the Hart Office Building, CNN said.
Senate Sergeant-at-Arms Terry Gainer said in an email there would be no mail delivery Thursday or Friday because of tightened security.
The Hill said staffers in Democratic Sen. Carl Levin's Saginaw, Mich., office reported a suspicious letter to law enforcement and the building was evacuated as a precaution.
The Hill said the letter addressed to the president was intercepted and sent to a Secret Service mail screening facility off-site from the White House for testing. The Secret Service has not said whether the letter tested positive for the poison.
"This facility routinely identifies letters and parcels that require secondary screening or scientific testing before delivery," the Secret Service said in a statement. "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."
Media reports said the White House letter -- like the letter sent to Wicker -- was postmarked Memphis, Tenn., and contained similar language.