The note was recovered Friday, the official said.
"Jiah Khan's mother has handed over the note to us along with her statement. It will be scrutinized and legal opinion will be sought," Additional Commissioner of Police (West) Vishwas Nagre-Patil told IANS.
Sooraj is the son of actor-couple Aditya Pancholi and Zarina Wahab. A day after 25-year-old Jiah Khan's suicide June 3, Sooraj was quizzed for several hours by police and allowed to go.
Police said they will seek expert help to verify Jiah's handwriting in the letter, a notarised copy of which has been handed over to investigators.
It is not known when Jiah Khan had penned the note and how it was discovered by her family six days after her death.
Last Monday around midnight, Jiah was found hanging in her home in Sagar Sangeet building in the posh Juhu area of north-west Mumbai where she lived with her family.
The six-page note details Jiah's strained relationship with Sooraj, alleging that he had cheated her though she trusted him.
"By the time you will be reading this letter, probably by then I won't be there in this world," the note is reported to have said, indicating that she had planned to end her life.