New Delhi, July 28 (ANI): Politicos on Monday expressed concern and asked the Mumbai Police to take precautionary measures after Mumbai Police Commissioner Rakesh Maria received a threat letter. The letter challenged the police to avert a terror attack. The letter received on July 25 said, "Stop us this time if you can, you were lucky in 1993." As a precautionary measure, all anti-terror cells in Mumbai have been put on high alert.