How many Americans are killed by police every year? Who are they? Are the incidents justified? Nobody knows the answers to those questions. Some 18,000 law enforcement agencies are asked to report to the FBI, but it’s voluntary. They don’t have to if they don’t want to, and different agencies use different standards to measure performance. Two US senators are proposing new rules for accountability, but there’s dispute about whether that’s a good idea. After a flood of stories about police killings, the Washington Post and the Guardian US have compiled statistics of their own — with results that are very different from those of the FBI. Would an accurate national database help to improve law enforcement or generate unwarranted federal power over local police?