An apparatus and method for penetrating bone marrow is provided. The apparatus may include a housing such as a handheld body, a penetrator assembly, a connector that releasably attaches the penetrator assembly to a drill shaft, a gear mechanism, a motor and a power supply and associated circuitry operable to power the motor. The penetrator assembly may include a removable inner trocar and an outer penetrator or needle. It may also include a grooved trocar that allows bone chips to be expelled as the apparatus is inserted into bone marrow. Various connectors are provided to attach the penetrator assembly to the drill shaft.