Reports indicate that the preliminary investigation by the CBI has found nothing incriminating against the former minister in the murders of Balipur village head Nanhe Yadav, his brother Suresh and Deputy Superintendent of Police Zia-ul-Haq.
Sources in the CBI said the summons were issued to "clear some doubts". The former minister has been asked to present himself at the camp office of the CBI. The notice was sent to Raja Bhaiyya's houses in Kunda and Lucknow late Thursday.
Raja Bhaiyya has been mentioned as a conspirator in the murder of the police official by Haq's widow, Parveen Azad, a medical student.
Officials say the former UP minister would be interrogated on his whereabouts at the time of the triple murder. He would be grilled on any "insights" into the role of his close aides and a driver, who have been arrested in the case.
Earlier, the agency grilled his cousin and former MP Akshay Pratap Singh alias Gopalji.
The investigating team of the CBI has also questioned Kunda Nagar panchayat head Gulshan Yadav, representative of the former minister, Hari Om Srivastava, driver Rohit Singh and legislator Vinod Saroj.
Haq's widow recently visited Kunda to take back her husband's belongings from his official residence, and expressed satisfaction at the probe so far. She said she expected investigations to be "free, fair and without any political influence."
The family of village pradhan Nanhe Yadav has alleged, meanwhile, that the CBI was trying to "water down" the case.
The CBI, earlier this week, sought the transfer of four station house officers at Kunda and of other police officials outside the area, citing "political influence".
Following this, the state government transferred 108 policeman out of Pratapgarh district.