Karna was informed of the means of Abhimanyu's death by the preceptor himself. While Abhimanyu was fighting with his sword, it was Drona who cut-off that sword in twain.
उपदिष्टो वधोपायः कर्णस्य गुरुणा स्वयम्। व्यायच्छतश्च खड्ड्रेन द्विधा खङ्गं चकार ह॥