The Syringe is Dr. Werner Klopek's primary weapon.
Dr. Klopek can be seen threatening Ray Peterson in the ambulance before pulling out the hypodermic needle, presumably filled with poison. He tells Ray that he let him keep the femur, but he wants the skull, attempting to stab and kill him with the syringe. Luckily, Ray manages to overpower Dr. Klopek.
The syringe was presumably used to poison The Knapps.