Police on Friday released three sketches of those suspected to have killed Manish Mishra, Prime Minister Atal Bihari Vajpayee's grandnephew.

Three persons detained on Thursday were let off after questioning as they were found to be innocent, Additional Director General (Railways) V K Bhalla said in Lucknow.

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The police had received some clues, which were being verified, he said.

Bhalla, who is conducting an inquiry into the case, said he had submitted his preliminary report to the government.

The theory that the killers could be candidates returning from an army recruitment drive was also being looked into, he added.

Bhalla said the government could also announce cash reward to nab the culprits.

Mishra, grandson of Vajpayee's sister Vimla Mishra, and two of his friends had tried to prevent the killers from teasing some girls travelling in the Chhattisgarh Express.

The assaulters, who were allegedly in an inebriated state, threw out Mishra and his friends.

Mishra, a BSc student in Mathura's BSA College, was killed while his friends were injured.