The case and Google’s role in it underscore a tension between some Silicon Valley companies and the government over national security surveillance and the secrecy surrounding it. Google has been pushing back against data-gathering tools called national security letters in two federal courts since this spring. Such letters, known as NSLs, allow the Federal Bureau of Investigation to demand account information and other data, but not the content of calls and emails. They typically come with a strict gag order, and companies cannot even acknowledge they receive them.