Police stations across the city have been alerted against possible attacks by accomplices of Aftab Ansari, sentenced to death in the American Center firing case.

A message from Lalbazar on Thursday asked the officers-in-charge of all 47 police stations to intensify vigil in their areas, especially around vital installations.

The move follows the detention of four youths for shouting slogans in favour of Ansari, hours after a city civil and sessions judge pronounced the sentence on Wednesday.

?The four were picked up from Mominpur and Beniapukur. They were released after interrogation,? said an officer.

The special branch, too, is fearing a retaliatory strike by the Ansari gang. ?We are in touch with the Central intelligence agencies and are sharing information with them,? said an state intelligence officer.

?Our units in the border districts have been asked to step up vigil. The porous points on the border are one of the most frequented routes used by underworld elements to enter or leave the country,? he added.

Security has been tightened around Writers? Buildings. ?All gates are being manned by personnel of the Radio Flying Squad, who are frisking all employees and visitors,? said an officer.

From Wednesday night, vigil at all entry and exit points of the city has been strengthened. ?Barricades have been set up on the Park Circus-EM Bypass connector, Rashbehari connector, Vidyasagar Setu and Howrah bridge. We are checking all vehicles entering or leaving the city,? he added.

The beef-up has been extended to the Metro Railway and Howrah and Sealdah stations, where plainclothesmen have been deployed in large numbers.

?There is a clear instruction that we interrogate anyone we find moving around suspiciously,? said an officer of Jorasanko police station.

The police in adjoining states, too, have been alerted. ?We have contacted senior officers in Bihar and Jharkhand. Ansari and his men had their hideout in Hazaribagh,? said an officer.