They also expressed suspicion about the involvement of school authorities in "destruction of evidence" and sought that the top management officials of the school be questioned by the CBI in the matter.
"We demand strict punishment for the class XIth student. The (Juvenile Justice) Board also has the option of having him tried as an adult. The CBI has apprehended him based on evidence and we will assist the CBI and the court to ensure he is not allowed to go scot-free, is tried as an adult and hanged till death," Sushil Tekriwal, the lawyer reresenting the victim's family, told reporters.
They also alleged a larger conspiracy behind the murder and said that they want the CBI to file a charge sheet soon.
Tekriwal also said that the agency had yet not given a clean chit to the Pinto family the trustees of Ryan International Group.
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"There is a deep-rooted conspiracy that has been hatched on the part of various people including the student who has been apprehended. The legal battle is going to be long but we are waiting for the CBI to file the charge sheet in a time- bound manner," he said.
"The kind of story which had come from the police initially was not believable. However, the CBI has apprehended this class XIth student and the reasons given by them justify it.
"However, all those whose negligence led to my son's murder should also be questioned. They should be given strict punishment so that it sends out a positive message that no one can go scot-free after committing such a heinous crime," he said.
In a sensational twist to the case, the high school student, who is about 16-years old, was apprehended late last night for allegedly killing his junior inside the school, said CBI spokesperson Abhishek Dayal said.
Pradyuman was found dead with his throat slit by a sharp-edged weapon on the morning of September 8.
The CBI did not find any evidence against bus conductor Ashok Kumar, who was the Gurgaon Police's sole accused in the gruesome killing.
It was the same knife seized by the Gurgaon Police, he said.
According to the agency, the Class 11 student, believed to be weak in studies, allegedly slit Pradyuman's throat to get the school to declare a holiday in order to defer a scheduled parent-teacher meeting (PTM) and an examination.
The CBI was able to piece together elements of the crime by analysing the CCTV footage, scientific and forensic examination, analysis of the crime scene and by questioning students, teachers and staff of the school, an official said.