Dehradun, Nov 28 : Police Department officials have tightened the security here, after some well-known organisations and public schools in the region received threatening letters from alleged terrorists.

The top brass of the police has sounded a high alert over the past two days after nation''s prime petroleum exploring agency, the Oil and Natural Gas Commission (ONGC) and the Doon School received threatening letters.

The ONGC had received a letter on Wednesday (November 25) that had threatened of serial blasts in the city and had bore the names of dozen institutions and offices as targets.

Doon School, too, received a photocopy of the letter received by ONGC.

The management of the prime residential school handed over the letter to the police and the security has been further beefed up.

The letters were posted from Chennai and Patna.

Security arrangements at public places, including bus stands, railway stations, district court campus and other crowded places, have been enhanced with posse of security personnel positioned at various vantage points in the city.

Besides the ONGC and the Doon School, security has also been tightened at other vital institutions like the Indian Military Academy, Rashtriya Indian Military College and the other schools.

Abhinav Kumar, Senior Superintendent of Police (SSP), Dehradun brushed aside speculations by saying that this work has been done by some miscreants and not by any terrorist organisation, national or international. Yet all precautions have been taken, he added.

"We take every threatening letter very seriously. These letters too have been taken very seriously. Our investigations are on and we stepped up security around the major crowded places and institutions in the city," said Abhinav Kumar, Senior Superintendent of Police, Dehradun.

R K Jasbir Singh, a retired Lieutenant General felt that Dehradun has caught the eyes of the militants because it is an open place where floating and migrant population is increasing.

"We need an efficient faculty of intelligence department here. There is no vigilance over here as to who is entering the state with what intentions. Here migrant labourers come. They may be local labourers or labourers from outside," said R K Jasbir Singh, a retired Lieutenant General of Indian Army.

Armed police personnel are manning pickets at various places and at certain specific locations; special commando outfits have been deployed. (ANI)