Google CEO Sundar Pichai. Reuters

Apple, Microsoft, Amazon, and Cisco have filed an amicus brief in support of Google, after a Pennsylvania court ruled that the company had to hand over emails stored overseas in response to an FBI warrant.

An amicus brief is filed by people or companies who have an interest in the case, but aren't directly involved. In this case, it's in Silicon Valley's interest to keep US law enforcement from accessing customer data stored outside the US.

It isn't clear what data Google might have to hand over and, last month, the company said it would fight to the order.

In the brief, the companies argue: "When a warrant seeks email content from a foreign data center, that invasion of privacy occurs outside the United States — in the place where the customers’ private communications are stored, and where they are accessed, and copied for the benefit of law enforcement, without the customer’s consent."

They claim that handing over foreign data "invites" other countries to demand emails from US citizens, stored on US soil, in the same way.

They wrote: "Our sister nations clearly view US warrants directing service providers to access, copy, and transmit to the United States data stored on servers located within their territory as an extraterritorial act on the part of the US government."

Their argument hinges on the court's interpretation of the US Stored Communications Act (SCA), and whether it applies to data stored outside the US. According to the companies, it's for Congress rather than the court to decide whether the SCA applies overseas.

They also referenced a similar case won by Microsoft in January. The company refused to hand over emails belonging to the non-US citizen stored on Irish servers, and the US government lost an appeal to have the case reheard.

According to The Register, Google said last month it would appeal the Pennsylvania court's decision. A spokesman said: "The magistrate in this case departed from precedent, and we plan to appeal the decision. We will continue to push back on over-broad warrants."