A new report concludes that a Department of Homeland Security pilot program improperly gathers data on Americans when it requires passengers embarking on foreign flights to undergo facial recognition scans to ensure they haven't overstayed visas.

The report, released on Thursday by researchers at the Center on Privacy and Technology at Georgetown University's law school, called the system an invasive surveillance tool that the department had installed at nearly a dozen airports without going through a required federal rule-making process.

The report's authors examined dozens of Department of Homeland Security documents and raised questions about the accuracy of facial recognition scans. They said the technology had high error rates and were subject to bias, because the scans often fail to properly identify women and African-Americans.

"It's telling that D.H.S. cannot identify a single benefit actually resulting from airport face scans at the departure gate," said Harrison Rudolph, an associate at the center and an author of the report.

"D.H.S. doesn't need a face-scanning system to catch travelers without a photo on file," he added. "It's alarming that D.H.S. still hasn't supplied evidence for the necessity of this $1 billion program."

Homeland security officials said the program was necessary and fulfilled a decades-old congressional requirement to prevent foreign visitors from overstaying their visas.