Complex sentence: The Court further held that the decision of counsel in defending a client facing the death penalty must be based either on a thorough investigation of all possible mitigating factors or on professional judgment providing sound reasons for limiting the investigation.
Split into simpler sentences: The Court further held that the counsel's decision in defending a client facing the death penalty must be based on a thorough investigation of all possible mitigating factors. Failing that, it must be based on competent professional judgment providing sound reasons for limiting the investigation.