Nearly 19 groups of Amazon shareholders have expressed reservations over sales of the company’s Rekognition service to law enforcement, NBC and CNN report.

In a letter addressed to Amazon CEO Jeff Bezos on Friday, a copy of which was provided to NBC by the American Civil Liberties Union (ACLU), the undersigned warn against potential abuses of the facial recognition technology. And they point to recent scrutiny of Facebook over its data privacy policies, which have negatively impacted its stock.

“While Rekognition may be intended to enhance some law enforcement activities, we are deeply concerned it may ultimately violate civil and human rights,” the shareholders wrote. “We are concerned the technology would be used to unfairly and disproportionately target and surveil people of color, immigrants, and civil society organizations … We are concerned sales may be expanded to foreign governments, including authoritarian regimes.”

The shareholders, which include advocacy organizations like the Social Equity Group and Northwest Coalition for Responsible Investment, join the ACLU and nearly 70 other groups protesting the online retailer’s practices. In a separate letter sent to Bezos on Monday, the ACLU argued that Amazon shouldn’t provide facial recognition systems to the government.

News that Amazon had supplied U.S. law enforcement with computer vision technology broke in May, when the ACLU published freedom of information requests showing that the company’s Amazon Web Services (AWS) division worked with the city of Orlando, Florida and the Washington County (Oregon) Sheriff’s Office to deploy Rekognition. At the time, both offices said that they weren’t using the system to perform real-time facial tracking.

The bulk of the controversy stems from research showing that facial recognition systems are susceptible to bias. A 2011 study found that systems developed in China, Japan, and South Korea had more trouble distinguishing between Caucasian faces than East Asians, and a separate study showed that algorithms from security vendor Cognitec performed 5 to 10 percent worse on African Americans than on Caucasians.

In an email statement provided to VentureBeat last month, Amazon said that it requires customers to “be responsible” when they use Amazon Web Services and Rekognition. “When we find that AWS services are being abused by a customer, we suspend that customer’s right to use our services,” an AWS spokesperson said. “Amazon Rekognition is a technology that helps automate recognizing people, objects, and activities in video and photos based on inputs provided by the customer.  For example, if the customer provided images of a chair, Rekognition could help find other chair images in a library of photos uploaded by the customer.”

Amazon didn’t immediately respond to a request for comment. We’ll update this article when it does.