The Gurgaon Police's Special Investigation Team or SIT detained the gardener yesterday.
"The SIT does not want to leave out a single piece of evidence which can be important and work as a key breakthrough for us to solve this case. With three days left for submission of charge sheet in court, we have got possible positive leads against the accused school bus conductor Ashok Kumar," a senior SIT officer told news agency PTI.
SIT teams yesterday searched the school here for clues to the crime while a CBSE panel also inspected the premises to examine loopholes in security arrangements.
Besides Harpal Singh, the SIT has questioned 17 persons including section incharge Anju Dudeja, suspended acting principal Neerja Batra, former principal Rakhi Verma, bus driver Saurabh Raghav, bus contractor Harkesh Pradhan and eight security guards.
The officer said questioning of all suspects who are on the police radar whether it is the suspended school principal, senior management officials, or staff members, is being conducted. "Some more arrests are likely to be made," he said.
The seven-year-old boy was found with his throat slit in the washroom of the school last Friday.