V Gokulraj, who graduated in engineering from a college in neighbouring Namakkal last year, was found dead on a railway track near Pallipalayam in Namakkal District on June 24.
Dalit groups allege that it to be a case of honour killing since he was in 'love' with a upper caste girl.
Police today taken into custody Chitra, the mother of Gokulraj, and members of Viduthalai Chiruthaigal Katchi and Dalit Panthers of India, immediately after they began an indefinite fast before the Salem Medical College Hospital demanding urgent police action against the culprits and registering murder case against them.
All of them were later let off, police said.
The postmortem on the body of Gokulraj was conducted by a team of doctors constituted by the Madras High Court.