Police said the officer was living in federal lodges and allegedly committed suicide. An investigation is underway.
Faisal was assisting investigation officer Asgahr Khan in the mega corruption scandal allegedly involving Ashraf.
"I have never come to grips with a mega corruption case like this one during my whole service in the bureau. A senior officer will be better suited to carry out an inquiry as sensitive as this," Faisal wrote in a letter dated Nov 13, 2012, a copy of which is with Geo News.
The Supreme Court earlier this week ordered the NAB to arrest Ashraf and 15 others in a case involving private power stations built to provide electricity to energy-starved Pakistan. Ashraf was minister for water and power at that time and is accused of receiving kickbacks.
In the letter, Kamran Faisal said that with just a basic one-month course under his belt, he was not capable of taking care of an investigation which demanded relevant experience.
"In my six-year tenure, I spent four and a half years in the bureau's IT department and monitoring cell. I have only dealt with a few illegal assets' cases. Thus I must be removed from the rental power projects case and moved to some routine cases," he said in the letter.