SP Asra Garg said that the letter mentions details of how the boy had fallen in love with the upper caste girl Divya and talks of his subsequent marriage to her.
The SP said they had recovered the letter based on 'secret' information.
He claimed the handwriting in the letter matched that in the notebooks used in college by Ilavarasan. The letter would be submitted in court along with the post-mortem report and other material evidence, Garg said.
The report came after Madras High Court directed the state government to preserve till Tuesday the body of Ilavarasan and give the deceased's father a copy of the post-mortem report along with video footage of the procedure, both of which have been done.
Ilavarasan, whose marriage to Divya triggered anti-Dalit violence in three villages of Dharmapuri district in November last year, was found dead along a railway track here on July 4, a day after the girl said she would never go back to him and instead stay with her mother.