At least 25 prominent artificial-intelligence researchers, including experts at Google, Facebook, Microsoft and a recent winner of the prestigious Turing Award, have signed a letter calling on Amazon to stop selling its facial-recognition technology to law enforcement agencies because it is biased against women and people of color. The letter, which was publicly released Wednesday, reflects growing concern in academia and the tech industry that bias in facial-recognition technology is a systemic problem. Amazon sells a product called Rekognition through its cloud-computing division, Amazon Web Services. The company said last year that early customers included the Orlando Police Department in Florida and the Washington County Sheriffs Office in Oregon. In January, two researchers at the Massachusetts Institute of Technology published a peer-reviewed study showing that Amazon Rekognition had more trouble identifying the gender of female and darker-skinned faces in photos than similar services from IBM and Microsoft. It mistook women for men 19 percent of the time, the study showed, and misidentified darker-skinned women for men 31 percent of the time. There are no laws or required standards to ensure that Rekognition is used in a manner that does not infringe on civil liberties, the A.I. researchers wrote. We call on Amazon to stop selling Rekognition to law enforcement.