The National Investigation Agency has arrested a self-styled sergeant major of NSCN-IM group in connection with the killing of Arunachal Pradesh MLA Tirong Aboh in May this year, an official spokesman said on Monday.
The 37-year-old Ellie Ketok was arrested on Sunday from Nagaland's Dimapur by a joint team of the terror probe agency and the Army, the spokesman said.
He said the accused was a part of killing team, and actually fired on Aboh and others on May 21 this year.
The NIA has earlier arrested five persons -- Sethok Kangnong, Napong Jenpi alias Jenpi, Jai Kishan Sharma, Luckin Mashangva and Yangte Josaham -- in this case.
The Arunachal Pradesh government had decided to handover the killing of Aboh, an MLA from Khonsa West, and nine others by suspected NSCN-IM militants to the NIA.
Nationalist Peoples' Party (NPP) MLA Aboh, his son and nine others were killed when suspected NSCN-IM militants opened fire at their vehicles near 12 Mile area in Tirap district on May 21.
The MLA along with his relatives was on his way to Khonsa from Dibrugarh in Assam when the attack happened.
The National Socialist Council of Nagaland (NSCN-IM) is in ceasefire with the Centre since 1997 and negotiations are on to resolve the decades-old Naga political problem.
A framework agreement was signed between the Centre and the NSCN (IM) in August 2015.