Google took almost three years to disclose to the open information group WikiLeaks that it had handed over emails and other digital data belonging to three of its staffers to the US government, under a secret search warrant issued by a federal judge.

WikiLeaks has written to Google's executive chairman, Eric Schmidt, to protest that the search giant only revealed the warrants last month, having been served them in March 2012. In the letter, WikiLeaks says it is "astonished and disturbed" that Google waited more than two and a half years to notify its subscribers, potentially depriving them of their ability to protect their rights to "privacy, association and freedom from illegal searches".

The letter, written by WikiLeaks' New York-based lawyer, Michael Ratner of the Center For Constitutional Rights, asks Google to list all the materials it provided to the FBI. Ratner also asks whether the California-based company did anything to challenge the warrants and whether it has received any further data demands it has yet to divulge.

Google revealed to WikiLeaks on Christmas Eve – a traditionally quiet news period – that it had responded to a Justice Department order to hand over a catch-all dragnet of digital data including all emails and IP addresses relating to the three staffers. The subjects of the warrants were the investigations editor of WikiLeaks, the British citizen Sarah Harrison; the spokesperson for the organisation, Kristinn Hrafnsson; and Joseph Farrell, one of its senior editors.

When it notified the WikiLeaks employees last month, Google said it had been unable to say anything about the warrants earlier as a gag order had been imposed. Google said the non-disclosure orders had subsequently been lifted, though it did not specify when.