JAKARTA, April 27 -- Indonesian environmental activist Muhammad Rosidi shudders as he recalls the horrific moment two men on a motorbike threw acid through the open window of his car as he was driving on Sumatra island in February.

He was instantly consumed by a burning pain as the corrosive liquid ate into his hands, legs and groin.

"I knew right away it was acid. It felt like being doused in boiling water," the 43-year-old told AFP.

No arrest has been made weeks after the attack that Rosidi is convinced was triggered by his campaigning against illegal tin mining and smuggling in the Bangka Belitung islands off Sumatra.

He is one of an increasing number of activists and government critics rights groups say are being targeted in ever-...