You should know if the government thinks it can deputize your email provider to scan through your messages.

Like most people, we were shocked at reports earlier this month that Yahoo scanned its hundreds of millions of users’ emails looking for a digital signature on behalf of the government. We join millions of Yahoo users in wanting to know how this happened.

Together with a host of other civil liberties groups – including the Center for Democracy and Technology, the ACLU, and the Sunlight Foundation – we sent a letter today asking Director of National Intelligence James Clapper to release information about the scanning, how the U.S. government justified such a privacy-invasive search, and whether the government has conducted similar searches.

The letter warns that Yahoo’s “massive scan of the emails of millions of people, particularly if it involves the scanning of email content, could violate the [Foreign Intelligence Surveillance Act], the Fourth Amendment, and international human rights law, and has grave implications for privacy.”

Although the letter calls on the government to release additional details about the Yahoo scanning order, a recent law passed by Congress requires its declassification and release, or, alternatively, that the government produce a declassified summary.

It’s crucial that Clapper follow through on his pledge for transparency and release information about how the U.S. government justified the email scanning under FISA, as has been reported. We need to know whether the Foreign Intelligence Surveillance Court has interpreted FISA – which authorizes targeted surveillance of certain foreigners’ (such as spies or terrorists) communications  – to mean that the government can conscript Yahoo into mass surveillance of all of its users’ emails.

The letter also calls on Clapper to acknowledge whether the scan also involved scanning the content of the emails, disclose the kinds of search terms used in this surveillance, and to identify when this kind of surveillance first started and the total numbers of times an order like this has been used.