Munir Thalib was a prominent and vocal critic of the Indonesian government, as well as its powerful military.
He was especially vocal during former leader Suharto's rule, but also continued his campaign after the president resigned in 1998.
Munir was murdered in September 2004, on his way to Utrecht University in the Netherlands to pursue a doctorate in international law and human rights.
He was found to have been poisoned by an off-duty pilot on the plane, who arranged for arsenic to be slipped into his drink during his flight transit in Singapore.
The pilot, Pollycarpus Priyanto, was found guilty of his murder in January 2008, and sentenced to 20 years in jail.
Indra Setiawan, the former CEO of Garuda Airlines, also received a year in jail for being an accessory.
But Munir's supporters have consistently claimed his trial failed to shed light on the alleged links between Pollycarpus and state intelligence.
They were therefore keenly following the trial of retired deputy intelligence chief Muchdi Purwopranjono, who was accused of being the real mastermind behind his murder.
Mr Purwopranjono was the most senior member of Indonesia's feared intelligence agency ever to be brought to trial.
Analysts say the fact he has now been acquitted not only throws into question the indictment of any more senior officials, it may also prompt Pollycarpus Priyanto to appeal.
Munir Thalib's death has posed as many awkward questions for the Indonesian authorities as he did himself before he was killed.
It was Munir's apparent fearlessness that made him a thorn in the side of the Indonesian authorities, and an icon for his supporters.
He was thought to have many enemies.
He said his office in Jakarta was attacked, and that he had lost count of the number of death threats made against him.
He brought attention to the methods used by the military in crushing dissent across Indonesia and in East Timor, and separatist movements in its Aceh and Papua provinces.
As a lawyer, he set up the Kontras human rights organisation that pursued the Suharto regime over the alleged kidnap and torture of opponents.
Munir won several awards for his efforts, including the renowned Right Livelihood Award.
He was educated in law at Brawijaya University in Malang, East Java, and started his career in 1989 as a legal aid officer.
He was 38 when he died.