The city police were placed on high alert after two TV news channels on Tuesday evening received an email threatening serial bombings in the metropolis at 10 pm on Tuesday. According to a channel office in Kolkata, the email said blasts would target locations like Park Circus, Park Street, Salt Lake City Centre, Science City, Salt Lake Tank No. 1 and 4, Alipore, Apollo Gleneagles Hospital and the Calcutta High Court.