The incident took place on Thursday and the shot was fired from a relative's gun.
The victim's family claimed that the killing was intentional.
The victim, identified as Dharmendra Singh, the groom's cousin, was a student of Class 10 and hailed from Rajasthan.
The incident took place at Sri Vrindavan Garden, said the police.
The police have booked two persons for the murder.
According to reports, the gun is owned by Gyanendra Singh, a retired Army man from Hathras, who had reportedly given it to the accused, Vivek, 19, after he insisted on seeing it closely.
After taking the loaded gun, Vivek pulled the trigger and a bullet hit Dharmendra in the chest.
The CCTV footage shows that the victim was standing with four other people, when the bullet hit him from a point-blank range.
Dharmendra was rushed to hospital but he succumbed to injuries on the way.
According to police, three bullets were fired and the third hit the boy.
The victim's elder brother, Anil Kumar, said that Dharmendra came to Agra on June 28 to attend the wedding.
"He came alone here and on Thursday, we were informed about his death. Someone had killed him deliberately," he said.
Arvind Kumar Nirwal, station house officer of Khandauli police station, said that an FIR had been registered against Vivek and Gyanendra Singh under IPC section 302 (murder) and 34 (acts done by several persons in furtherance of common intention) and further investigations were on.
The police are scanning the video footage from the wedding.