A Hyderabad-bound Indigo plane was grounded for nearly four hours after a bomb threat at the IGI airport in New Delhi and later cleared for take off last night after thorough security checks. <br/><br/>CISF officials at the airport said that the flight has been cleared after security checks and no explosive or ammunition has been found inside the plane. They said it is suspected that the call was made in a bid to delay the flight by a few hours.<br/>