Narcotics Control Bureau officials on Tuesday raided the home of former Central Bureau of Investigation additional director Upen Biswas, in connection with the drug racket unearthed in his house in Kolkata.

Additional director Nitai Patra, who led the NCB team, said they had searched Biswas' home with court orders.

It wasn't officially known if the NCB officers had found any incriminating evidence, but sources said 'some important documents' were recovered.

Asked if Biswas, who has not been charged with complicity in the racket that operated out of the ground floor of his residence, was now a suspect, Patra said: "No one, absolutely no one is above suspicion until the investigations are complete."

On May 17, NCB officials had recovered banned psychotropic substances from the ground floor of Biswas' house.

Five foreigners -- two Chinese and three Myanmarese -- were arrested from the house for possessing 24kgs of ephedrine hydrochloride valued at Rs 50 million and used in making a chemical substitute of heroin.

Biswas has said the five foreigners were not his tenants and the person, Siang Nang Thanga, he had rented out a part of his house, was missing.

NCB officials believe that the drug operations at Biswas' house were part of the Chinese Ah Hua cartel.

Besides the five foreigners, three other persons, including Thanga's wife, had been arrested.

Officials have interrogated Biswas several times. Their suspicion was raised after Biswas returned a consignment for Thanga from Singapore that came to his residence on May 22 without informing the authorities.

The consignment weighing more than 200kg, which was impounded by NCB at the Kolkata airport, was found to contain ephedrine hydrochloride.