New Delhi: Udaiveer 'Kala', a 42-year-old notorious criminal lodged in Tihar jail as a prime accused in the murder of former MLA Bharat Singh, died on Wednesday afternoon after a sudden deterioration in his condition. Police haven't ruled out foul play. Sources said, Kala, who was already admitted in the medical room of jail number 1, complained of breathlessness around 12.20pm. He was taken to DDU hospital where he died at 12.45pm. Officials said the prisoner had a history of cardiac problems and probably died of a heart attack. However, cops are also probing the homicide angle as Kala was on the radar of the Kishan Pehelwan gang ever since his dramatic arrest by the crime branch last year. Kala's family has alleged that he was killed inside the prison. A drug overdose or poisoning angle is also being probed. Bharat Singh, 38, Pehelwan's brother and a former INLD legislator from Najafgarh, was shot dead by six assailants in March last year while attending a function in the locality. The attackers had fired around 15-20 bullets, of which of six hit Singh, including two in the head. He was declared dead at Gurgaon's Medanta hospital. Kala, who had a running feud with the Pehelwan gang, was arrested for masterminding Singh's murder in April 2015 from Loni border. His nephew, Hemant, a key player in the conspiracy, is still absconding.