(KUNA) -- Security at the historical Taj Mahal in Northern Indian city of Delhi was Tuesday beefed up and every visitor is being scanned through electronic surveillance systems after officials received a letter threatening to bomb the monument. The letter contained a specific threat to blow off the Taj Mahal and the sender said he was linked to the Al Qaida, news agency Indo-Asian News Service reported.Security forces are guarding the outer circle of the monument and also keeping an eye on every visitor through close circuit cameras and other electronic surveillance systems. Due to the large numbers of foreign tourists, the security agencies are keeping high alert and are extra cautious at the Taj Mahal, the news agency reported.