Faisal was the assistant director of the National Accountability Bureau (NAB), assisting in the corruption scandal allegedly involving Ashraf.
He was found hanging from a ceiling fan in a house in Islamabad Jan 18.
The Supreme Court recently ordered the NAB to arrest Premier Ashraf and 15 others in the case involving 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.
The murder case was registered on a complaint lodged by NAB Additional Director (Coordination) Noman Aslam, the Dawn reported.
Aslam said the case was filed "on account of widespread rumours and suspicions" that Faisal could have been murdered.
Police would investigate the case as murder and if found to be suicide, they would investigate the circumstances in which Faisal committed it, advocate Chaudhry Ashraf Gujar said.
In case of suicide, investigators would probe who were responsible for it and under whose pressure Faisal took the extreme step.
If anyone is found involved in "creating the circumstances" in which Faisal committed suicide, he will face murder charges.
Media reports say Faisal had requested the director general of the anti-corruption agency to take him off the power plants case.
His relatives have alleged that Faisal's body bore torture marks on the arms, back and wrists.
They also said Faisal was frequently receiving threatening calls from unidentified people.
