United Nations officials called Thursday for an investigation into the death of a leading human rights activist who was found dead in Kinshasa on Wednesday. The activist, Floribert Chebeya, director of the national network of human rights organizations, was found dead in his car in a Kinshasa suburb after being called to an appointment with the police inspector general, Gen. John Numbi, on Tuesday evening. Mr. Chebeya, who campaigned to uphold constitutional rights and improve prison conditions, had been harassed by the authorities in the past and had been described as at risk by Amnesty International.