The 69-year-old, who also served as Nagaland governor and led the investigation into the Aarushi Talwar murder case, left behind a note saying he was "embarking on a new journey", officials said.
Kumar became CBI director in 2008, succeeding Vijay Shanker, in the midst of the agency's probe into the Aarushi Talwar murder case that hogged the headlines.
Ashwani Kumar's family has negated the possibility of foul play.
Former bureaucrats are pushing politicians from key posts, feel party loyalists.
Under Bharatiya Janata Party's attack over appointing former Central Bureau of Investigation Director Ashwani Kumar as Governor of Nagaland, the government on Monday said it was within its right to do so for which there have been precedents.
Former CBI Director Ashwani Kumar and Congress veteran S C Jamir were on Saturday appointed as governors of Nagaland and Odisha respectively.
Advocating the need for speeding up inquiries against officials with "doubtful integrity", Central Bureau of Investigation Director Ashwani Kumar on Wednesday blamed the "weak" criminal justice system for encouraging corrupt practices.
The CBI on Tuesday said it has not questioned any RSS functionary during its ongoing probe into Mecca Masjid and Ajmer blast cases.
Central Bureau of Investigation Director Ashwani Kumar on Monday denied that the premier investigative agency works under pressure from government but said shortage of staff was affecting its performance.
The United States Securities and Exchange Commission, the federal agency to regulate the securities industry in that country, has sought India's permission to probe the multi-million dollar Satyam fraud case.
CVC secretary K S Ramasubban on October one wrote to the CBI director Ashwani Kumar seeking 'a discreet field verification' on the allegations of RIL purchasing flats in Mumbai for oil regulator V K Sibal's daughter and incurring expenses over purchase of white goods for her.
There is a link among the blasts carried out at Ajmer Dargah, Mecca Masjid and Malegaon with self-styled right-wing group Abhinav Bharat believed to be behind them, Central Bureau of Investigation said on Monday.
Facing flak for alleged shoddy prosecution in the 1984 Bhopal gas leak case, the Central Bureau of Investigation has accepted that its prosecution was not good as compared to the accused, who brought a battery of lawyers to defend them.
The Central Bureau of Investigation has informed the Centre about its reservations on taking up the case of alleged rape and murder of two women in Shopian district of Jammu and Kashmir and it was upto the government to take a decision.
With economic offences seeing an increase, the CBI has written to the Institute of Chartered Accountants of India (ICAI) to probe the growing number of white collar crimes.
CBI director Ashwani Kumar said the Satyam fraud was unique, as it was the founder, and not an outsider, who is the accused. According to sources, the fraud involves an amount that is much bigger than the Rs 7,800 crore (Rs 78 billion) estimated originally. It could be close to Rs 10,000 crore (Rs 100 billion) based on over 7,000 fake invoices and forged documents retrieved by the agency.
The CBI team under Kumar had concluded that Aarushi and domestic help Hemraj were murdered by Krishna Thadarai- help at Talwar's clinic, his friend Rajkumar-- a domestic servant with Praful and Anita Durrani who were Talwars' friends, and Vijay Mandal, driver of Talwars' neighbour.