Calcutta police on Wednesday picked up four persons as it started a probe to identify some of those who had been involved in alleged acts of rioting outside the CBI office here following the arrests of two Bengal ministers in the Narada case on Monday.

The investigation came close on the heels of a letter from the CBI to the police bosses in Lalbazar, seeking details of the force’s deployment that had been made after the central agency had apparently informed the police about the ministers’ arrests and voiced their apprehensions of trouble outside its office.

Apart from Bengal ministers Subrata Mukherjee and Firhad Hakim, the CBI had on Monday arrested Trinamul Congress MLA Madan Mitra and former Calcutta mayor Sovan Chatterjee in the Narada case.

The quartet arrested from the city on Wednesday were allegedly present in the mob and had reportedly hurled stones and bottles at the central forces deployed to secure Nizam Palace on Monday. Three of them were arrested from parts of Kidderpore and one from Karaya an the basis of an FIR that the CBI had lodged with the Shakespeare Sarani police station.

The FIR blamed “unknown persons” for “rioting and unlawful assembly” on May 17 outside Nizam Palace, which houses the office of the Anti Corruption Branch of the CBI’s eastern zone.

“We are in the process of collecting more details based on our source network of those who had turned up outside Nizam Palace on Monday,” said a senior police officer of the Calcutta police. “More arrests are likely in this case.”

Sources said the central investigating agency had sought details like the strength of the force that had been deployed outside the CBI office, names of officers in charge of the police personnel and efforts to contain lawlessness that had played out for several hours, forcing the CBI to abandon the idea of presenting the four accused physically in a court.

The CBI’s contention is that it had informed Lalbazar about the arrests to be made and sought police support for rounding up the four leaders from their residences in south Calcutta.

The CBI also informed senior police officers about their apprehensions of a possible lawlessness as a fallout of the arrests.

“We mentioned the situation outside the Nizam Palace within hours of the arrests in our petition to Calcutta High Court,” said a senior officer of the agency.

In Lalbazar, police bosses said all arrangements had been made and adequate police personnel deployed to control the mob. Use of any force to restrain the mob would have been counter-productive, said one of them.

“Several senior officers had turned up and barricades were put up everywhere to ensure that none of central force personnel was injured,” said a senior IPS officer.

“There is no question of any slackness in arrangements since the chief minister of the state was present inside the building.”