When Janna Holm's grandmother Dorothy Ann died of brain cancer in 1996 she was unable to speak. But in the last two weeks of her life she covered 20 or more index cards with long unbroken strings of capital letters that made no apparent sense. Her family was convinced that she meant something by her writing but they could not decipher the mystery.

Dorothy Ann had led a quiet life in Minnesota. She married Lyle Holm, a former second world war submariner, in 1949 and had four children, one who died in infancy. After her marriage, she devoted herself to her children and, later, grandchildren. Janna remembers that her grandmother "never flew in a plane and never drove a car (scared of both), and hated having her photo taken".

Eighteen years after Dorothy Ann's death, Janna photographed one of the cards, which her father had kept, and uploaded it to the internet discussion site Metafilter. She thought they might refer to song lyrics, but within 15 minutes another user had spotted that one of these strings of capitals, "OFWAIHHBTNTKCTWBDOEAIIIHGUTDODBAFUOT", was an acronymic prayer: "Our Father, who art in heaven …"

Different readers joined in with their reconstructions and guesses and more and more sense emerged from the letters. Some of them were requests to God: "Please see that we are all happy and safe, praise and glory, amen. Please see that we are all in excellent health and please see that we are reunited in heaven. Please see that trouble, pain and injury (or illness) do not befall T, V, J, E, or any of us, praise and glory, amen."

Much of it, extraordinarily for a woman dying in pain, seems to have been thanksgiving. The last four have been reconstructed as: "Thank you almighty God for protection from grief. Thank you almighty God for listening to my prayers and answering them. Thank you almighty God for everything, amen. Thank you almighty God for everything, amen, amen, amen."

All of these readings are guesses, of course, but it seems clear that everything on the card was a prayer of some sort. Much of the document remains undeciphered, although one of the participants in the discussion has written a small program to turn the whole text of the authorised version of the Bible into an acronym to see if parts match against the cards. It's a kind of treasure hunt for the readers, but it's also an extraordinarily intimate glimpse into the mind of a dying woman.

Reading the dearest hopes of this extremely private woman doesn't, oddly, feel voyeuristic. But it does raise in the sharpest form the question of what prayer is. Dorothy Ann did not know when she wrote the cards that complete strangers would be reading them 18 years later, and discover what she meant. But even if we decipher every word correctly, we will never know the answer to the largest question: when she sent these thanks and pleas to God, was anyone, was anything, there to listen?