They were initially not taken seriously.However, two letters received at his office on Monday reportedly contained explicit threats to kill Ganguly and his wife and harm people close to him.The police have now started an investigation to identify the sender and find out the motive behind the threats. "We are taking the complaint seriously and have begun an investigation. "We have spoken to the people from the courier service and tracked down one mobile number. We are trying to spot it," the officer said.Kolkata Police are now checking whether all the letters were sent by the same person. Investigators are also trying to establish the reason behind the alleged threats.The police are examining the contents of the letters, their source and the courier trail as part of the probe.