In a news conference, Ashcroft said it had been found in a piece of luggage belonging to Mohamed Atta, 33, an Egyptian suspected of being one of the lead hijackers in the Sept. 11 attacks.
Atta, a pilot, was aboard American Airlines Flight 11, which hit the World Trade Center. Nearly 6,500 people are dead or missing in the attacks on the twin towers and the Pentagon and the crash of a fourth hijacked jet in Pennsylvania.
Cambridge University scholar Akbar Ahmed tells CBS News Correspondent Wyatt Andrews this kind of note indicates one mastermind shaping a twisted view of Islam.
"There is a kind of ideology here, however disjointed it is," he said.
Ashcroft said that two more copies of the letter were found. One was discovered in the vehicle parked by another hijacker at Dulles airport, near Washington. He said a third copy of essentially the same document had been found in the wreckage of the plane which crashed in Pennsylvania after passengers apparently struggled with the hijackers.
CBS News Correspondent Stephanie Lambidakis reports there are some differences between the translation of the letter by the Justice Department, and that which was printed in Friday morning newspapers: The letter appears to be written in classic Koran-style Arabic which is very difficult to translate.
The letter gives the hijackers step-by-step instructions for their suicide mission and preparing themselves spiritually for death, a law enforcement official said Friday.
The document also contained practical reminders to bring "knives, your will, IDs, your passport" and to "make sure nobody is following you."
"This letter is clear evidence linking the hijackers on three separate flights," Ashcroft said.
"Everybody hates death, fears death," according to a translation of highlights of the document obtained by The Washington Post.
"But only those, the believers who know the life after death and the reward after death, would be the ones who will be seeking death."
"You should pray, you should fast. You should ask God for guidance, you should ask God for help....Continue to pray throughout this night. Continue to recite the Koran.
"The time of judgment has arrived. Hence we need to utilize those few hours to ask God for forgiveness. You have to be convinced that those few hours that are left you in your life are very few. From there you will begin to live the happy life, the infinite paradise."
It said to "make sure that you are clean, your clothes are clean, including your shoes.
"Check all of your items your bag, your clothes, knives, your will, your IDs, your passport, all your papers. Check your safety before you leave...Make sure that nobody is following you."
The document found in Pennsylvania contained directives on what actions, thoughts and prayers should be undertaken in the final hours, according to The Dallas Morning News.
It instructs a follower, on the day of the attack, to "check your weapon," say morning prayers together, and, "If you take a taxi to the airport, when you arrive...smile and rest assured, for Allah is with the believers and the angels are protecting you."
The follower is told to pray as he sets foot on the plane and again as he takes his seat, remembering "It is a raid for Allah."
The book also contained this passage for comfort: "When the time of truth comes and zero hour arrives, then straighten out your clothes, open your chest and welcome death for the sake of Allah. Seconds before the target, your last words should be 'There is no God but Allah. Mohammed is his messenger.'"
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