New Delhi: The Indian Armed Forces have always been targeted by Pakistan's Inter Services Intelligence (ISI). Many espionage attempts have been thwarted successfully in the past. However, over the last few years, ISI may have managed to infiltrate the force, the ISI threat to the Indian armed forces is for real. Several ISI agents have been caught selling defence information in the recent past. In July 2006, the Indian Air Force (IAF) was shaken by the news of a possible terrorist infiltration. Media reports quoted a letter written in August 2005 by National Security Adviser M K Narayanan to state chief secretaries, which mentioned such a possibility. In November 2005, IAF was once again warned of a possible infiltration by ISI backed Lashkar militants. Narayanan's letter was written at a time when several Army and Air Force personnel were being arrested for spying on ISI's behalf. In July 2005 Kolkata police had arrested a former IAF officer along with his son, who was a soldier in the Army. The father-son duos were part of an ISI module, active for many years. Months later, in February 2006, two other ISI agents from the same group were arrested. But the infiltration did not remain confined to the IAF. Just weeks before the letter was made public in June 2006, the J&K police arrested three soldiers and two policemen in Rajouri. The five arrested were not only accused of providing logistic support to ISI backed militant groups, but also for leaking sensitive information. During the past few years, the Indo-Nepal border has also become a hub for ISI activity. In May 2006, police arrested a former Lance Naik from Army Communication wing, in Bihar. He allegedly passed defence related information to ISI agents at the border. Although, Intelligence agencies have maintained a tight grip on ISI activities targeting Armed forces, there have undoubtedly been many chinks in their armour.