- Congress rebel Kalikho Pul was found hanging in his home today
- In February, he became chief minister with the backing of the BJP
- Last month, the Supreme Court said his appointment was illegal
Mr Pul left a diary, which is being examined by the police. Chief Minister Pema Khandu, who took over from him last month, has ordered an inquiry.
As a coffin was brought to the bungalow in Itanagar where the body was found, angry supporters set fire to it, alleging that it was hardly befitting a former chief minister.
Mr Pul's supporters also attacked the homes of Deputy Chief Minister Chowna Mein and another minister.
Last month, the Supreme Court for the first time removed a government in power and ordered that Arunachal Pradesh be restored to the charge of the Congress.
Kalikho Pul was with the Congress till late 2015 when he rebelled along with a large section of legislators from the party against then chief minister Nabam Tuki; in February, he became chief minister with the backing of the BJP.
That appointment was illegal, the Supreme Court ruled, as the Governor had no power to summon the assembly bypassing the state government.
The rebels returned when the Congress, in an attempt to win them over, dropped Nabam Tuki and picked Mr Khandu as chief minister. Chowna Mein was the leader who negotiated the return of the rebels.
At Mr Khandu's oath-taking ceremony, Mr Pul, who was seated in front row, had told NDTV: "I always follow my heart and did what I thought was right. Now the court order has come and I am following the court order."
Sources say Mr Pul felt isolated after the lawmakers who had backed him went back to the Congress, completely abandoning their rebellion.
Top Congress leaders including Sonia and Rahul Gandhi condoled his death. "We appreciate the great dignity with which he conducted himself, the balance he showed in trying times...everyone needs to introspect and learn a lesson," said party spokesperson Abhishek Manu Singhvi.