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Earlier this week, we learned that Yahoo had been scanning its entire email service for the FBI using software that was developed by Yahoo in-house. Now a new report reveals how the company developed the scanning system and what kind of information the U.S. government was looking for.

According to The New York Times, Yahoo actually modified an existing system originally designed to scan its emails for spam, malware and child pornography. The software was apparently re-purposed to search for a specific digital signature that the U.S. government believed was tied to a state-sponsored terrorist group. Any emails flagged by the system were sent directly to the FBI, though the email-scanning software is apparently no longer active.

The report doesn't reveal what the signature was or how many emails were sent back to the FBI. It does note that it's unusual for the U.S. government to demand access to an entire email system, rather than targeting specific users. Several anonymous tech companies also told The New York Times they did not receive similar requests.

Yahoo was barred from revealing the order and the company has been unable to clarify exactly what happened. However, CEO Melissa Mayer's decision to comply with the government's request apparently pushed Chief Information Security Officer Alex Stamos to resign from the company last year.