Here are the latest developments in this story:
Mehdi was arrested from his one-room flat in Bangalore's Jallahalli in the early hours of Saturday. After watching his interview with UK-based Channel 4, the authorities traced him by the T-shirt he was wearing. The investigation took 14 hours.
During his initial questioning, Mehdi said Indian Muslims were incapable of Jehad, so he was concentrating on Muslims living aboard. Most of his 17,000 followers on Twitter were English-speakers from Europe.
The youth was not in direct contact with jihadis and had no role in recruitment, police said. He was a self-radicalised person spreading jihadi propaganda through Twitter - mainly translating Arabic tweets into English and uploading data and videos.
Although Mehdi was in contact with English-speaking IS men on Twitter, he was careful to hide his identity from them. Also, there was no evidence to show any IS funding came his way.
Police said Mehdi got interested in the developments in middle-east - Turkey, Syria, Lebanon, Israel, Gaza strip, Egypt & Libya - since 2003. He used to work during the day and surfed internet on a 60 GB monthly plan late into nights.
The UK based Channel 4 News said, "His tweets were seen two million times each month, making him perhaps the most influential Islamic State Twitter account."
Mehdi had told the channel that he had considered joining the "jihad" in Syria and Iraq, but could not do so owing to his family responsibilities.
Mehdi had moved to Bangalore in 2011 and was earning an annual salary of Rs 5.3 lakh. His father, a retired employee of the West Bengal State Electricity Board, said "My son can never do this, I will never believe this."
While most of his tweets have been deleted and the account has been closed, police say they have found no anti-India activity or tweets so far. There is nothing to suggest that he had wanted any terror attack in India.
Mehdi has been accused of waging war against the nation, waging war against a friendly nation and under sections of the IT Act. Police say he has no previous criminal record. His cellphone and laptop were seized during his arrest.