Complex sentence: The letter says that the KGB maintained contact with Rajiv Gandhi, who expressed his gratitude to the KGB for benefits accruing to his family from commercial dealings of a controlled firm, and a considerable portion of funds obtained from this channel were used to support his party.
Split into simpler sentences: The letter says the KGB maintained contact with Gandhi, who expressed his gratitude to the KGB for benefits accruing to his family from commercial dealings of a controlled firm. A considerable portion of funds obtained from this channel were used to support his party.