The Supreme Court today termed the Central Bureau of Investigation (CBI) a "caged parrot" that "speaks in its master's voice".
A bench headed by Justice RM Lodha also asked the government whether it was contemplating a law to make the working of the CBI independent and insulate it from extraneous intrusion and interferences.
The apex court questioned the credibility of the CBI probe into the allocation of coal blocks and asked for a thorough and qualitative investigation.
Expressing displeasure at the government's interference in the coal allocation probe report, the court said, "the heart of the report was changed on the suggestions of the government officials".
"It is a caged parrot speaking in its master's voice," it said.
"It's a sordid saga that there are many masters and one parrot," it added.
The court's observations came on CBI director Ranjit Sinha's second affidavit filed on Monday, stating that Law Minister Ashwani Kumar and senior officials of the Prime Minister's Office and the coal ministry had made certain changes in the coal block allocation probe report.
Asking the government to make the investigating agency impartial, the apex court said it needs to be ensured that the CBI functions free of all external pressures.
"If the CBI is not made independent, we will step in," it observed.
It also said that the "job of CBI is not to interact with government officials but to interrogate to find the truth".
The court said the agency "must know how to stand up against all pulls and pressures by government and its officials."