Security agencies in Amritsar were put on high alert on Wednesday after three police stations in the Commissionerate received a threat email warning of grenade attacks on police stations and IED blasts on railway tracks ahead of Independence Day celebrations.The email was sent to the official email IDs of airport, D Division and E Division police stations. The sender claimed affiliation with an alleged terrorist organisation and threatened attacks on police stations and railway infrastructure in Punjab before August 15.The message specifically named Amritsar, Ludhiana and Bathinda in Punjab, besides Ambala, and warned of blasts on railway tracks. It also threatened grenade attacks at police stations during specified hours and urged people not to travel by train or send children to schools and Independence Day functions.Police officials have taken the threat seriously and launched an investigation to ascertain the source and credibility of the email. Technical teams are examining it for clues regarding its origin and possible links to earlier threat messages.A senior police official confirmed receiving the threatening emails. He said the cyber cell has traced certain digital footprints and a technical probe is underway.Security arrangements around police stations, railway stations and other vital installations are being reviewed and strengthened ahead of Independence Day, he added.The development comes amid heightened security across Punjab in view of the August 15 celebrations and recent incidents involving suspected cross-border terror networks.