The WTC death toll is still thought to exceed 6,000
The Washington Post newspaper published extracts from the document, which was found in the abandoned luggage of Mohamed Atta, after he and others crashed the first airliner along with its passengers and crew into the World Trade Center.
US Attorney-General John Ashcroft said it was a "disturbing and shocking view into the mind-set of these terrorists".
In the letter, the hijackers are urged to bear in mind "the battle of the prophet... against the infidels as he went on building the Islamic state".
Mohamed Atta himself says he hopes to "glorify God in every possible way".
Happiness ahead
"Remind yourself that in this night you will face many challenges but you have to face them and understand it 100%," he is alleged to have writen.
"Obey God... and don't fight among yourselves where you become weak... God will stand with those who stood fast."
The hijackers are told to pray and fast, continually reciting the Koran, putting behind them "the time of fun and waste".
They should look ahead, instead, to living "the happy life, the infinite paradise".
"You will wish for death before you meet it if only you know what the reward after death will be," the document says.
The hijackers are urged to "keep a very open mind, keep a very open heart" as they act.
Clean shoes
"Check all of your items - your bag, your clothes, knives, your will, IDs, your passport, all your papers," Atta advises.
"Check your safety before you leave... Make sure nobody is following you."
The hijackers should make sure they are cleanly dressed, including their shoes.
The document contains a section entitled "When You Enter The Plane".
According to the extracts published, it contains no practical advice but, instead, calls on "the God of all earth and skies" to aid the hijackers.
"Oh God, open all doors for me..." Atta writes.
"I am asking you to lighten my way. I am asking you to lift the burden I feel."
The extracts end with the words: "We are of God, and to God we return".