SHILLONG, May 21 (PTI)-The Army and the police have been alerted and security measures tightened up all along the railway track in Assam, following a train blast, near Chaparamukh junction on the North-East Frontier Railway early this morning, in which at least four persons were killed and several others injured. A high-level conference of senior Army police and Railway security officers will be held here in Friday to take stock of the situation arising out of the explosion. According to Mr. B.K. Dey, Inspector-General of Police. Assam, the explosion was a “definite act of sabotage”. Asked about the possible hand behind the explosion, Mr. Dev said: “Our first thoughts go to Nagar rebels.” He, however, added that “investigations are in progress”.