Nina Lindsey, an Amazon Web Services spokeswoman, said in a statement that the company’s customers had used its facial recognition technology for various beneficial purposes, including preventing human trafficking and reuniting missing children with their families. She added that the A.C.L.U. had used the company’s face-matching technology, called Amazon Rekognition, differently during its test than the company recommended for law enforcement customers.

For one thing, she said, police departments do not typically use the software to make fully autonomous decisions about people’s identities. “It is worth noting that in real-world scenarios, Amazon Rekognition is almost exclusively used to help narrow the field and allow humans to expeditiously review and consider options using their judgment,” Ms. Lindsey said in the statement.

She also noted that the A.C.L.U had used the system’s default setting for matches, called a “confidence threshold,” of 80 percent. That means the group counted any face matches the system proposed that had a similarity score of 80 percent or more. Amazon itself uses the same percentage in one facial recognition example on its site describing matching an employee’s face with a work ID badge. But Ms. Lindsey said Amazon recommended that police departments use a much higher similarity score — 95 percent — to reduce the likelihood of erroneous matches.

Facial recognition — a technology that can be used to identify unknown people in photos or videos without their knowledge or permission — is fast becoming a top target for civil liberties groups and privacy experts.

Proponents see it as a useful tool that can help identify criminals. It was recently used to identify the man charged in the deadly shooting at The Capital Gazette’s newsroom in Annapolis, Md.

But civil liberties groups view it as a surveillance system that can inhibit people’s ability to participate in political protests or go about their lives anonymously. This month, Microsoft said the technology was too risky for tech companies to deploy without government oversight and called on Congress to regulate it.