An intraosseous access system (100, 1200) comprising: a needle assembly comprising: a needle hub; and a needle attached to the needle hub and defining a lumen, the needle being configured for insertion into a mammalian body and comprising a distal face formed as a bevel at an angle relative to a central longitudinal axis of the needle and configured to pierce or cut through tissue; an elongated instrument being an obturator positioned within the lumen of the needle and substantially blocking the passage into the lumen of the needle, the elongated instrument comprising a distal tip, a proximal portion that defines a contact surface, and a recess that is laterally recessed relative to the contact surface; and a shield coupled with each of the needle hub and the elongated instrument, the shield being configured to transition from an unlocked state in which the shield permits the elongated instrument to translate relative to the shield to a locked state in which the shield attaches to a distal end of the elongated instrument to restrict access to the distal tip of the elongated instrument, the shield comprising an arm (162, 163) comprising a contact region (190, 191) that is configured to translate along the contact surface of the elongated instrument to maintain the shield in the unlocked state as the elongated instrument is retracted from the lumen of the needle, and wherein at least a portion of the contact region (190, 191) of the arm (162, 163) is configured to enter into the recess of the elongated instrument to transition the shield to the locked state. a needle assembly comprising: a needle hub; and a needle attached to the needle hub and defining a lumen, the needle being configured for insertion into a mammalian body and comprising a distal face formed as a bevel at an angle relative to a central longitudinal axis of the needle and configured to pierce or cut through tissue; a needle hub; and a needle attached to the needle hub and defining a lumen, the needle being configured for insertion into a mammalian body and comprising a distal face formed as a bevel at an angle relative to a central longitudinal axis of the needle and configured to pierce or cut through tissue; an elongated instrument being an obturator positioned within the lumen of the needle and substantially blocking the passage into the lumen of the needle, the elongated instrument comprising a distal tip, a proximal portion that defines a contact surface, and a recess that is laterally recessed relative to the contact surface; and a shield coupled with each of the needle hub and the elongated instrument, the shield being configured to transition from an unlocked state in which the shield permits the elongated instrument to translate relative to the shield to a locked state in which the shield attaches to a distal end of the elongated instrument to restrict access to the distal tip of the elongated instrument, the shield comprising an arm (162, 163) comprising a contact region (190, 191) that is configured to translate along the contact surface of the elongated instrument to maintain the shield in the unlocked state as the elongated instrument is retracted from the lumen of the needle, and wherein at least a portion of the contact region (190, 191) of the arm (162, 163) is configured to enter into the recess of the elongated instrument to transition the shield to the locked state.