A post-mortem was conducted in Mumbai's JJ Hospital.
Police said Jiah's body was discovered by her mother. No suicide note was found. Her domestic help, watchman and neighbours are also being questioned, police said.
Jiah's mother Rabiya Amin told the police that Jiah had gone to Hyderabad on June 2 for an audition which did not go well.
Her last rites have not yet been performed because the family is waiting for her friends and relatives to arrive from London, where she was born and brought up.
Jiah, real name Nafisa Khan, made her Bollywood debut in Ram Gopal Varma's Nishabd in which she played a teenager in love with her best friend's father, played by Amitabh Bachchan. The film released in March 2007 and received mixed reviews for its provocative content but Jiah was noted for her confidence, attitude, and sex appeal. She was also nominated for the Filmfare Best Debut award.
She then appeared with Aamir Khan and Asin in in A R Murugadoss's Ghajini (2008), the Hindi remake of the director's Tamil film of the same name. Jiah played the pivotal role of a medical student who investigates and then befriends a businessman with amnesia.
Her last film was Sajid Khan's Housefull in 2010 with Akshay Kumar, Deepika Padukone, Riteish Deshmukh and Lara Dutta. She had a two-film deal with UTV Motion Pictures and a three-film deal with Ghajini producer Madhu Mantena.
On Twitter, colleagues reacted with shock and grief.
One of the first reactions was from her first co-star Amitabh Bachchan who tweeted: T 1128 -WHAT ...!!! Jiah Khan ??? what has happened ? is this correct ? unbelievable !!!
Director Ram Gopal Varma, who gave Jiah her first break in Nishabd, tweeted tellingly: The last time I met her jiah told me that everyone around her makes her feel like a failure
Bipasha Basu said: #Rip Jiah Khan!God bless her soul!
Shahid Kapoor, with who Jiah had been cast in Chance Pe Dance before being replaced by Genelia D'Souza, tweeted: Shocked n saddened to hear about jiah khan... Very disturbing... RIP... may her soul rest in peace