Mir was taken into custody by the cops after tracking down his mobile number which was listed in the diary of the slain militants, intelligence sources said.
They said that Mir, a resident of Uri town along the Line of Control in Baramulla district, had guided the militants from the border town to the city.
Mir's questioning had led the investigators to the arrest of a Pakistani militant, involved in the attack, from Chattabal area of the city on Thursday.
Based on disclosures made by Mir, a team of SOG sleuths has been rushed to Uri town and more arrests are likely, the sources said.
Police has set up special nakas to keep a check on people leaving and entering Uri, they added.
Police officials, however, are tight lipped about the developments of the case.