The head of Poland's National Library says the letters that a Polish Cardinal and later Pope John Paul II wrote to a woman philosopher document a "difficult and courageous" friendship of 32 years and suggest she could have been in love with the Catholic Church leader.

The library bought hundreds of letters for a "seven-digit" amount in 2008 from the woman, Polish-born American thinker Anna-Teresa Tymieniecka, and is planning to publish them.

The head of the library, Tomasz Makowski, said Tuesday the letters were written by the cardinal, and later pope, from 1973 until shortly before his 2005 death. He said they hint that Tymieniecka, who died in 2014 aged 91, might have been in love with him, but he kept their relationship on a purely friendly and intellectual level.