Virtually ruling out immediate arrest of Raghuraj Pratap Singh alias Raja Bhaiya booked in the murder of a DSP, Uttar Pradesh Police today said it needs to be established first who committed the crime before the conspiracy angle can be probed.

The police also said that the post-mortem report of slain DSP Zia-ul-Haq, Circle Officer of Kunda in Pratapgarh district from where Raja Bhaiya is a five-time Independent MLA, has shown only one bullet injury and the bullet is untraceable.

As the Akhilesh Yadav government came under intense pressure for action against Raja Bhaiya over the killing of Zia in mob violence in Kunda on Saturday, a letter seeking a CBI probe into the incident has been sent to the Centre.

"The state government has sent a letter to the Centre in connection with the CBI inquiry into the Pratapgarh incident," ADG(Law and Order) Arun Kumar told reporters.

44-year-old Raja Bhaiya, who has been booked under Section 120B IPC(criminal conspiracy), yesterday resigned from the state Cabinet.

To a question on when action would be taken against Raja Bhaiya, Kumar said "before the criminal conspiracy, the crime has to be established first, only then further action is taken. As and when we get evidence further action will be taken. The government has already decided to hand over the case to the CBI".

Kumar also said the police was yet to find an eyewitness to the incident and also yet to identify the murder weapon.

He said all the four FIRs filed would be handed over to the CBI and that its probe will bring out the truth.

Rajya Bhaiya has been named in one of the FIRs following a complaint by the slain officer's wife Parveen Azad.

Two persons including Raja Bhaiya's supporter Guddu Singh have already been arrested after lodging of four FIRs.

Raja Bhaiya, who has a criminal record and has been seent to jail before, claimed he is being framed by his political opponents and welcomed the CBI probe. A former Jails minister, he also claimed that Guddu Singh was in Lucknow during the mob violence.

Zia was killed by a mob in Pratapgarh when he had rushed there during the course of his duty after a village head was shot dead over a land dispute.

According to the post mortem examination, the ADG said only one bullet injury was found on the body of the slain DSP.

"The report states that one bullet was fired on the chest from the back, which passed through the front, so its bore was not known, and we are searching the bullet.

"Until the bullet is found it cannot be ascertained whether it was fired from the service revolver or not," the ADG said adding that there was no other gun shot injury on his body.

The post mortem examination on the pradhan Nanhe Yadav has found two bullet injuries, of a .315 bore gun and a country made weapon, while his brother Suresh Yadav's body bore pellet injury marks on groin, the ADG said.

The ADG said the police does not generally use the bullets of the kind recovered from the body of the Yadav brothers and the police is also saying that it did not fire and the matter is still under investigation.

The officer said that statements of those arrested have been taken but refused to divulge the details.

Giving an account, the ADG claimed,"the station officer has said that because of the darkness, they could not gauge the situation that it would not be right for them go without force."

"While the other policemen ran on the right side, the CO went on the left where his body was later found with marks showing that it was dragged on the ground," he said.

Till the case is handed over to the CBI, Special Task Force of the UP police is also assisting the local police in the investigations, he said.