A high alert was sounded in Punjab after the arrest of another four alleged members of a terrorist module that tried to blow up an oil tanker last month. It was the fourth terror module backed by Pakistan Intelligence agency ISI busted in the state over the last 40 days. Chief Minister Amarinder Singh has ordered the police to alert, especially in view of the opening of schools and other educational institutions as well as the coming festival season and next year's assembly polls.High security will be put in place in busy areas, including markets and sensitive installations.Earlier, one person was arrested in what has come to be known as the tiffin bomb case. State police chief Dinkar Gupta said two Pakistan-based terrorists, including a Pakistani Intelligence Officer, have also been identified.The attack took place on August 8 and the police was told that a tanker parked at a petrol pump in Ajnala -- around 250 km from Chandigarh -- has caught fire.CCTV footage from the filling station showed four unknown persons came to the place twice and then placed a suspicious package on the fuel tank of the tanker. Around 11.30 pm, two of the suspects returned, and within a minute, there was an explosion and the fire.Preliminary investigations revealed that an Improvised Explosive Device, placed in a tiffin carrier, was used.The police chief said a Pakistani intelligence officer, identified as Qasim, and International Sikh Youth Federation chief Rode had promised to transfer over Rs 2 lakh to the terrorist module for carrying out the blast. Investigations into the financial angle are in progress.In August, the police found a sophisticated tiffin bomb in village Daleke in Amritsar. It contained around 2-3 kg RDX and had 3 different trigger mechanisms.Around August 15, the police had arrested two alleged terrorists and recovered a cache of weapons, including hand grenades.Another terror module was busted on August 20, the police said, and a tiffin bomb and other arms including hand grenades, detonators, RDX and guns were recovered from Gurmukh Singh Rode and Gagandeep Singh.