Petr Zelenka (born February 27, 1976) is a Czech serial killer. Zelenka, a nurse in Havlíčkův Brod, 100 kilometres (62 mi) southeast of Prague, murdered seven patients by lethal injection, and attempted to kill 10 others between May and December, 2006. He killed with a hidden vial of heparin — a blood-thinning drug causing internal bleeding when administered in large doses. Zelenka has already confessed and been taken into custody. His lawyer has been quoted as saying that Zelenka may have killed on an impulse to "test" doctors in Havlíčkův Brod, in the belief they were not good enough to discover the truth. In February 2008, Zelenka was convicted of killing seven patients and attempting to kill 10 others, and sentenced to life imprisonment.