New Delhi: The Intelligence Bureau has issued an alert for Jharkhand, Chhattisgarh and Bihar about a possible Naxal attack. The alert says Naxals are observing the Independence Day as black day and could launch a major attack.br br According to the alert the Naxals could attack train lines in Dhanbad and Giridih districts of Jharkhand while security forces could be targeted in Chhatisgarh's Darba ghati where Congress leaders were killed in an ambush in June.