Google is facing renewed questions about its handling of subscribers’ private information following the revelation that it waited six months after the lifting of a gagging order to alert WikiLeaks that emails and other data belonging to its employees had been passed to the FBI.

It was disclosed last month that Google had cooperated with federal agents after the search giant was served with secret warrants demanding that it hand over all emails and IP addresses relating to three WikiLeaks staffers. The warrants named the British citizen Sarah Harrison; senior WikiLeaks editor Joseph Farrell and the spokesperson for the organisation, Kristinn Hrafnsson.

The three were informed by Google on 23 December that their personal data had been disclosed to the US government. Google said that it had handed over “responsive documents” to detectives, though it did not specify the nature of the data it had divulged or how much.

The initial warrants were issued in March 2012, and Google said the three-year pause before WikiLeaks was notified was explained by a gagging order imposed by a federal judge that prohibited the tech company from discussing the matter with anybody, including the targeted individuals. But new information obtained by the investigative reporter Alexa O’Brien shows that the warrants were in fact unsealed by the US district court in Alexandria, Virginia, on 15 May last year – six months before WikiLeaks was approached.

WikiLeaks has long suspected that Google disclosed the existence of the warrants in the week of Christmas in order to bury uncomfortable material in a slow news cycle.

In a statement, a Google spokesperson said the company’s policy was to “tell people about government requests for their data, except in limited cases, like when we are gagged by a court order, which sadly happens quite frequently”. The spokesperson said that they had challenged many orders relating to WikiLeaks that in turn had led to disclosures to the individuals concerned.

“We’ve also pushed to unseal all the documents related to the investigation. We continue to argue for surveillance reform which would enable us to be more transparent.”

Michael Ratner, WikiLeaks’ US-based lawyer, said that Harrison, Hrafnsson and Farrell should have been given notice of the warrants soon after they were unsealed last May. “The question I have as their attorney is: what caused this six-month delay in notifying our clients of these search warrants?”

A clue to why the warrants were issued in the first place is given by the fact that they came from a court in the eastern district of Virginia – the same jurisdiction in which a grand jury was convened as part of a criminal investigation into WikiLeaks. The investigation, which is believed to be ongoing, related to the leaking of a vast quantity of secret US government documents by the Army soldier Chelsea Manning.

This week it was announced that Manning, who is serving a 35-year sentence for the leak in Fort Leavenworth, Kansas, will write an opinion column for the Guardian.