Google says the FBI is monitoring the Web for potential terrorist activity. But it can't say how extensive the surveillance is.

As part of the Google Transparency Report, the Internet giant this week released data on so-called National Security Letters -- official requests for data under the Patriot Act passed after the September 11, 2001 attacks.

But Google said it was only allowed to provide broad ranges of numbers: in the years from 2009 to 2012, for example, it received between zero and 999 requests.

The requests affected between 1,000 and 1,999 accounts, except in 2010 when the range was 2,000 to 2,999 accounts, Google said.

"You'll notice that we're reporting numerical ranges rather than exact numbers. This is to address concerns raised by the FBI, Justice Department and other agencies that releasing exact numbers might reveal information about investigations," said a blog post from Google law enforcement director Richard Salgado.

Salgado added, "We're thankful to U.S. government officials for working with us to provide greater insight into the use of NSLs."

The numbers, while inexact, were believed to be the first data from a private company about the National Security Letters which have been criticized by civil liberties groups for giving the government too much power to conduct surveillance without a warrant.

The Electronic Frontier Foundation calls the letters "dangerous" and has challenged the authority along with the American Civil Liberties Union.

One inspector general review found "serious deficiencies" in the FBI's handling of the process and noted that the letters concerned tens of thousands of US citizens and non-Americans.

EFF said public records have documented "the FBI's systemic abuse of this power."