Radley Balko covers it:
Last week, an undercover New York City police officer participating in a drug buy shot and killed 49-year-old Shem Walker during an altercation at Walker‰s home in Brooklyn. Police say Walker, described by family and neighbors as an ex-con who had reformed, apparently thought the officer was a drug dealer or a vagrant. When the officer didn‰t respond to Walker‰s verbal demand to leave his property, apparently because he was wearing earphones to monitor the drug buy, Walker tried to forcibly remove him from Walker‰s front stoop. The two got into an altercation. A second undercover officer then joined the fight, at which point the first officer shot and killed Walker.