Those scans were reportedly looking for a single piece of digital ID linked to a foreign government labeled as a "state sponsor of terrorism," Reuters reports. That means they were hunting for emails from Iran, Syria or Sudan.
"As legislators, it is our responsibility to have accurate information about the intelligence activities conducted by the federal government," according to the congressional letter. "Accordingly, we request information and a briefing as soon as possible for all members of Congress to resolve the issues raised by these reports."
This letter comes amid increasing scrutiny for Yahoo and the US Intelligence community regarding the practice. Legal experts have expressed concerns about whether these scans constitute a violation of the 4th Amendment (the one protecting against unreasonable search and seizure). The debacle has also caused Verizon to slash a billion dollars off of its offer for the company, if it doesn't sink the deal altogether.