WASHINGTON — Investigators seeking the source of the ricin detected two weeks ago in a Senate office building have raised the possibility that the positive test that forced the evacuation of lawmakers and staff members may have been caused by paper byproducts, not the deadly poison, NBC News has learned.
Sources familiar with the investigation, speaking on condition of anonymity, said that federal agents have found no source for the powder found in the mailroom of Senate Majority Leader Bill Frist’s office.
In addition to the apparent absence of a means of delivery, suspicions that the positive test might have been a false alarm have been heightened by the fact that the amount of the powder initially believed to be ricin was very small, precluding the performance of a potency test by the labs that received samples.
A possible explanation for the positive reading also has emerged. The sources said that investigators have determined that non-toxic byproducts of the castor bean plant — the raw material for ricin — are sometimes used in making paper. Because tests performed on congressional mail are highly sensitive, they could have picked up minute traces of products derived from the castor plant — not actual ricin, according to this theory.
Video: Questions surround ricin scare Investigators are only beginning to explore the false positive theory and are considering a wide range of possibilities in a case that so far has no solid leads, the sources said.
The ricin was discovered on a mail-sorting machine in Frist’s office in the Dirksen office building on Feb. 2. But law enforcement officials said at the time that no letter or note was found indicating how it got there.
The discovery of ricin prompted the closure of three Senate office buildings, the Dirksen, Russell and Hart buildings, and decontamination procedures for staff and Capitol police officers who were at the scene. Ricin is a highly toxic substance with no known antidote. It can easily be made from castor beans.
Officials said early this month that investigators were examining whether there is any link between the toxin found in Frist’s office and that mailed in two letters by a self-styled “Fallen Angel” angered by new federal rules requiring longer rest periods for truck drivers.
Those letters were found Oct. 15 at a mail facility in Greenville, S.C., and Nov. 6 at an offsite location where mail is processed for the White House. The “Fallen Angel” author, claiming to be a tanker fleet owner, threatens in both letters to “start dumping” more ricin if the new rules are not repealed.
New House, Senate procedures
House and Senate officials announced Wednesday they are initiating new mail inspection procedures, including the opening of all mail at an off-site location. Under new protocols, all letters will be removed from envelopes, reinserted and resealed after being found safe, House Sergeant at Arms Bill Livingood and Chief Administrative Officer Jay Eagen said in a letter to House members.
The Senate sergeant at arms office said similar measures would be adopted on the Senate side.
Rep. Dennis Kucinich, D-Ohio, who is running for the Democratic presidential nomination, has requested both bodies to suspend the new procedures, which he said raised privacy concerns. He also questioned having the testing outsourced to a private corporation.
“I believe these new procedures fundamentally damage the integrity of the chain of communication between constituents and members of Congress,” Kucinich wrote.
Rep. Bob Ney, R-Ohio, chairman of the House Administration Committee, said he planned to expand an experiment in which mail is scanned before reaching the Capitol and then sent electronically to lawmakers’ offices.
NBC News' Pete Williams and the Associated Press contributed to this report.