The police identified the deceased as Prateek Singh, a resident of Prem Nagar in Shuklaganj. His body was found in Basdhanaunder late Tuesday, the Gangaghat Kotwali police said.
Singh, who was posted as an Airman in Jammu, had come to his native place on June 11.
Police said, on Monday night around 9 p.m. after receiving a phone call, he had left home for somewhere.
When he did not return home, the family members searched for him and later lodged a complaint with the Gangaghat police.
On Tuesday, villagers found a body lying on the road in Basdhana village under Hajipur police out-post with bullet wounds and alerted the police.
"Bullet injuries were found in his eyes, besides an empty cartridge of 315 bore was recovered from near his body," ASP Unnao Shashi Shekhar Singh, said.
The body has been shifted to the district hospital for post-mortem.
"On the basis of a complaint, we have detained his relative Vinay Soni, who is also an Air Force employee and a resident of Unnao's Achalganj, for questioning," the ASP said.
"Prateek's family has accused Vinay of calling him over some pretext and later murdering him. We are also trying to ascertain the call detail record of all those persons who were in touch with the deceased during the last few hours prior to his killing and also scanning CCTV footage," the officer said.