The National Security Agency can gather and keep information about US citizens from online communications without a warrant for up to five years — as long as they get it by “accident”. That’s the revelation in the latest leaked documents in the NSA scandal.

To date the story has largely centered on the ability of the NSA to access US-based Internet firms to gather information on non-US citizens without a warrant or court order. Government officials have admitted the capability (available through a program known as PRISM) but have maintained security staff don’t have the power to look at domestic communications without a warrant.

Now the Guardian newspaper in the UK has published two leaked documents from the Foreign Intelligence Surveillance Court, the judicial body that authorizes the NSA to use tools such as PRISM. The documents detail the way the NSA is allowed to gather data from foreigners and the procedures it must follow to minimize data collection from US citizens.

The documents reveal a major loophole to the general “foreigners only” principle. It turns out that security staff have the authority to read the content a message gathered from a US machine or telephone in order to check whether or not the sender or recipient is actually in the US and thus should be excluded from further surveillance.

However, if the security staff are reading any “inadvertently acquired” communications within the US, they are allowed to keep and use the contents if they contain any useful intelligence relating to criminal activity or a threat of harm either to people or property.

In other words:

• the spies aren’t allowed to access domestic communications without a warrant; but
• they are allowed to read a communication to check if it is domestic; and
• if they find any suspicious info while carrying out this check, they can keep and use the information, despite not having a warrant.

The Guardian also reports that the legal authority that the NSA has to follow the procedures detailed in the document comes from a Foreign Intelligence Surveillance Court, order that is just one paragraph long and extends the authority for up to one year at a time.