The Delhi high court on Wednesday acquitted suspended IPS officer Ravi Kant Sharma and two others accused of killing journalist Shivani Bhatnagar in 1999.
Former Indian Police Services officer RK Sharma is released from Tihar Jail in New Delhi.
But a bench of justice BD Ahmad and justice Manmohan Singh - which took 10 months to write the verdict - upheld the life sentence awarded to the fourth accused, 'hired killer' Pradeep Sharma.
Shivani, a journalist with the Indian Express, was strangled with a wire and then stabbed 10 times with a kitchen knife on January 23, 1999 at her East Delhi apartment.
Delhi Police counsel Pawan Sharma said the acquittals would be challenged before the Supreme Court.
"The verdict is strange especially as the hired killer has been convicted," he said.
The prosecution said Shivani first met Sharma when he was an officer on special duty in the PMO during IK Gujral's tenure as prime minister and they allegedly had an affair.
Sharma showed some classified documents to her, including details of the St Kitts case.
The prosecution said he later decided to get her killed, since Shivani allegedly threatened him with exposure if he didn't marry her. But the prosecution failed to establish motive or criminal conspiracy.
Reversing a lower court's March 2008 conviction order, the bench said there was no evidence on record to prove Pradeep acted at the behest of Sharma and the other two, Satya Prakash and Shri Bhagwan.
It said: "Did he (Pradeep) act alone? Did he act at the behest of RK Sharma or did he act at the instance of someone else? These are questions we can't answer on the basis of the material before us.
"Judges, like other human beings, have suspicions. But judges, unlike others who are free to arrive at their own conclusions, cannot and do not convict on the basis of mere suspicion."
The court ordered the release of Sharma, in jail for the last nine years, and the other two if they were not required in any other case.
It also dismissed a Delhi Police petition against the acquittal of two other accused, Ved Prakash Sharma and Ved alias Kalu, by the trial court.
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