Several activists and academics on Saturday wrote to the Tamil Nadu government expressing their concern for the safety of an environmental rights activist who went missing a week ago.

S Mugilan reportedly “disappeared” after he addressed a press conference in Chennai on February 15. He had accused senior police officials of orchestrating violence in Thoothukudi during last year’s anti-Sterlite protests, in which at least 13 people were killed in police firing.

“We are writing to urge you to ensure the safe return of Mr Mugilan, an environmental rights activist who went missing at around midnight on 15 February,” the letter addressed to the Tamil Nadu chief secretary and other senior police officers said. “His disappearance barely half-a-day after highlighting the alleged role of senior police officers in the killings of 14 persons in a police action against anti-Sterlite protestors in Thoothukudi last May is deeply disturbing.”

Mugilan’s abrupt disappearance and the silence of the state government raise grave concerns about the state of civil liberties and human rights in Tamil Nadu, said the letter. “We... express our deep concern and anxiety over Mugilan’s safety,” it added. “We hope that the government of Tamil Nadu and the state police will work speedily to ensure his safe return to his family.”

The signatories include lawyer Prashant Bhushan, Swaraj Party President Yogendra Yadav, retired Madras High Court Justice D Hariparanthaman, writer V Geetha, and activists Medha Patkar, Nityanand Jayaraman, Suhas Kolhekar and Ashish Kothari.

At the press conference, Mugilan had alleged that the police, in connivance with the management of Sterlite, were behind the violence that took place on May 22. He specifically named Indian Police Service officers Shailesh Kumar Yadav and Kapil Kumar Saratkar. While Yadav was the inspector general of police of south zone at the time of the incident, Saratkar was the Tirunelveli deputy inspector general of police. Both were transferred out of the area in June.

Mugilan was reportedly last seen by a few of his friends at the Egmore Railway Station in Chennai. Four days later, Amnesty India had demanded that state authorities initiate an investigation to find him. “The Tamil Nadu government must ensure that dissenting voices in the state are protected, and that human rights defenders, activists, journalists and lawyers can work without fear,” Amnesty India had said in a statement.