This disclosure describes a device for obtaining a blood sample from an intruder at a crime scene to be later analyzed for DNA. A projectile containing a blood-extracting needle for penetrating the skin of said intruder is mounted through a protective chamber and connected to a blood reservoir in the chamber interior. A vacuum is held in both the reservoir and the chamber. The chamber vacuum is released by forward movement of the chamber as it slides within a casing following further penetration of the needle. The vacuum release acts to reduce the ambient pressure within the forward portion of the casing, thus assisting in propelling the needle into the intruder. As the needle reaches maximum extension, a loaded spring within the casing is released, propelling the needle and reservoir away from the intruder.