The incident occurred when the girl along with her brother was going to visit a doctor, and died of bullet injuries on the spot, they said.
The girl was allegedly raped on June 6, 2011, by one B K Singh, Manager of a college, police said, adding the case was going on in the local court, where she had to appear as witness after two days.
The victim's family members have alleged that she was killed upon the directions of the rape accused, they said.
Police is trying to workout the case and the guilty will not be spared, Additional Superintendent of Police Sunil Kumar Singh said.
Meanwhile, the victim's brother said, "My sister was raped and the case was going on. After holi, he (accused) got bail and started threatening us during her wedding.
"My mother reported it to police after which the accused threatened to shoot her. Pradeep shot at my sister at her in-laws' place."
The girl, who belonged to Mehmoodpur village, was married in Pritampur village and both these villages come under the same police station, a police official said.
"The victim's husband had come to take his wife. When they came out far away from the village, a person belonging to the village shot at the woman. According to investigation so far, a rape case was registered against the culprit and that appears to be the enmity," the official said.
Reacting to the incident, senior Congress leader and MLA from Lucknow Cantt constituency, Rita Bahuguna Joshi said that all women in the nation are shocked.
"Until now, there were rapes and cases were being subverted and inquiries were not done properly but now we find that the girls are being shot dead. We have always been arguing that there should be a fast track court and good judgement, but we find that the police doesn't really investigate the case properly," she said.
By the time the case goes to court, the girl's family already comes under a lot of pressure. In UP, the lawlessness is up to such an extent that now these murders are happening so blatantly, she further said.
Lawyer and activist Abha Singh said, "Do you think that Mulayam Singh and the CM office will do something for women safety when Singh has come and made such derogatory statements about women. Who trivialises rape by saying it's just a mistake when it is a serious crime under the Indian Penal Code.
"In fact, these rapists are emboldened under the present government of Akhilesh Yadav because they get political patronage and protection. The police is clearly told not to arrest criminals and that is why women are so unsafe in UP.”
"Home Minister Rajnath Singh, who belongs to UP, should look into his state and see why women are not safe, why a rape victim is openly shot at. It clearly shows that law and order has broken down."