However, the government informed the court that there is 'strong' prima facie evidence against the Marxist veteran in the case relating to assignment of about 2.33 acres in Kasaragod to his close relative T K Soman, when he was chief minister.
In the conversation, leaked to the media and aired by TV channels, Natarajan is heard asking Kunju, who is investigating the case, to go soft on the Marxist leader.
In an affidavit, Achuthanandan alleged the leakage was part of a conspiracy to implicate him and said government was bound to explain how the conversation recorded by the IO had come into the hands of the media.
The vigilance director has ordered a probe into the lobbying to drop Achutanandan's name from the FIR.
Meanwhile, Director General of Prosecutions T Asaf Ali, in a statement filed in the case, said it was very evident that there was an attempt of the Information commissioner to interfere in the investigation at the instance of the petitioner, hoping to escape from the clutches of the law.
There was a 'strong' prima facie case against Achuthanandan and other accused, he stated.
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The affidavit by Achuthanandan and statement by DGP was filed in a petition seeking to quash the case registered against the Marxist leader by the Kasaragod Vigilance DYSP and to hand over investigation to a senior IPS officer.
Earlier, Achuthanandan's request to change the IO in the case relating to assignment of about 2.33 acres of land to T K Soman, his close relative, was declined by vigilance director.
Achuthanandan contended that the event which took place during the pendency of the case justifies the charges against the IO.
The media had given wide coverage to the telephonic talk recorded by the investigating officer. It would not have reached the hands of the media except through him, Achuthanandan said.
Leakage of the audio when the case is being considered by the High Court 'casts serious doubts about the fairness of the investigating officer,' he said.