The attack on Agnes Kharshiing was the second such incident on a social activist in the district.
The prime accused in the attack on Meghalaya rights activist Agnes Kharshiing and her aide, has surrendered before the police in East Jaintia Hills district, a police officer said on Thursday.
Nidamon Chullet, who is a leader of the ruling National Peoples Party in East Jaintia Hills district and the prime accused in the assault of the two women, surrendered on Wednesday.
Another accused, identified as Cheerful K Ryngkhlem, also surrendered on the same day, police added.
Earlier, the police had arrested six persons in connection with the attack.
The RTI activist had suffered injuries after she was allegedly assaulted by a group of people suspected to be belonging to the coal mining mafia in East Jaintia Hills district on November 8.
Ms Kharshiing and her aide Amita Sangma had gone to the area to capture photographs of illegal coal mining and were stopped by the some people on their way back.
The 58-year-old activist was stopped near Tuber Sohshrieh, from where she was later found in an unconscious state. Her companion too was injured.
Ms Sangma had written to Governor Tathagata Roy seeking the arrest of Chullet, who she had claimed, was the "mastermind" of the attack.
The two had demanded a Central Bureau of Investigation (CBI) inquiry into the case to identify all the accused and trace the people who were sponsoring the alleged illegal mining racket in Meghalaya.
Chief Minister Conrad Sangma had condemned the incident and asked the police to nab the culprits.
Violence in any form would not be tolerated and the government would take all necessary steps to ensure that those behind the attack were arrested, the chief minister had said.
The state government had also approved a proposal for a judicial probe into the matter, but has not come out with an official notification yet.