Adjust font size:

WASHINGTON (CNN) -- Cookies mailed to the U.S. Supreme Court last year contained enough rat poison to kill all nine justices, retired member Sandra Day O'Connor said at a conference last week.

Barbara Joan March, a 60-year-old Connecticut woman, was sentenced last month to 15 years in prison. She sent 14 threatening letters in April 2005 -- each with a baked good or piece of candy laced with rat poison -- to a variety of federal officials: the nine Supreme Court justices; FBI Director Robert Mueller; his deputy; the chief of naval operations; the Air Force chief of staff and the chief of staff of the Army.

March pleaded guilty in March to 14 counts of mailing injurious articles.

March's plea received little public attention until O'Connor discussed it last week. (Watch how plot to poison justices was revealed -- 1:52)

"Every member of the Supreme Court received a wonderful package of home-baked cookies, and I don't know why, (but) the staff decided to analyze them," the Fort Worth Star-Telegram quoted O'Connor as saying at the legal conference November 10 in the Dallas area. "Each one contained enough poison to kill the entire membership of the court."

The letters did not seem to pose much of a real danger since the threatening note told the recipients the food was poisoned. In court papers submitted with the plea agreement, prosecutors said each of the envelopes contained a one-page typewritten letter stating either "I am" or "We are" followed by "going to kill you. This is poisoned."

Supreme Court spokeswoman Kathleen Arberg said the poison packages never reached the chambers of the justices.

All mail sent to the court is screened, and there has been heightened security since anthrax-laced letters were sent to members of Congress and the media in 2001. The Supreme Court also received some suspicious packages at the time, forcing it to shut down for a short period of time. Those packages turned out to be harmless.

Authorities said March included fake handwritten signatures of the purported senders of the letters whose names and return addresses were typed both in the body of the letter and on the envelopes.

Prosecutors said the purported senders live throughout the United States, and were connected to March in various ways, including being classmates, a former co-worker and a former roommate.

Prosecutors said handwritten documents recovered in March's apartment "reflect that she engaged in considerable planning in order to prepare and send the letters," including making a detailed list of the purported senders and an apparent to-do list.

CNN Supreme Court Producer Bill Mears contributed to this report .