Six people who were arrested for their alleged involvement in violence during the protests against Sterlite Copper’s smelter in Tamil Nadu’s Thoothukudi district were booked under the National Security Act on Monday, reported PTI.

Thirteen people agitating against the expansion of the Vedanta Group’s Sterlite Copper plant in the coastal city were killed in police firing on May 22 and May 23. Several people were arrested for rioting, burning of vehicles on the Tuticorin collectorate campus, stone pelting and damaging public property.

Kaleel Rahman, Mohammed Younis, Mohammed Ishraf, Velmurugan, Saravanan and Sottaiyan of the Makkal Adhigaram group were arrested on May 22 and lodged in Palayamkottai Central Prison in Tirunelveli district, reported The News Minute. They were detained under NSA on the orders of Home Secretary Niranjan Mardi based on recommendations made by the district superintendent of police and the district collector.

“They have been involved in many cases of violence and were booked under provisions of NSA,” said Thoothukudi Superintendent of Police Murali Ramba, according to The News Minute. Under the NSA, a person can be detained for up to 12 months to prevent him from acting in any manner prejudicial to national security.

Makkal Adhigaram, however, said that the police have no proof to make such a charge. “So if protesting against Sterlite jeopardises national security, does it mean that the company is now the nation?” said spokesperson for the group Marudhu, according to The News Minute.

For more than two decades, activists in Thoothukudi have accused Sterlite of contaminating the region’s air and water resources, causing breathing disorders, skin diseases, heart conditions and cancer among the residents. Since February, there have been large-scale protests in Thoothukudi, where Sterlite ran a copper smelter with the capacity to produce 4.38 lakh tonnes of anodes per annum, or 1,200 tonnes per day.