By Mark Ward Technology Correspondent, BBC News website

The US government wants to know what people were looking for

At first glance it is hard to understand why the US Government's Justice Department wants Google to hand over a week's worth of search data.

To begin with, this would be a huge amount of information to pore through. In an average week, Google handles between 500 million and a billion searches.

While this is far less than the US government's request for 60 days worth of data, it is likely to produce a huge file tens, if not hundreds, of gigabytes in size.

At the same time, the US government wants a random sample of one million websites that people have searched for.

One of Google's defences against the Justice Department request is that it already has data from other search firms such as Yahoo, AOL and MSN.

However, government lawyers argue that the way Google ranks and classifies the data it returns to searchers will prove particularly useful for its purposes.

Essentially it wants data from search engines to prove how easy it is to stumble over porn on the net. If it can prove this the result might be onerous regulation for many websites.

In court documents the US government said it had tried to generate the same information using the Internet Archive website but did not get the results it wanted.

Dangerous game

It is well known that net service firms and search engines do hand over records to law enforcement agencies who need the data for ongoing criminal investigations.

In many cases data found in computer memory caches or searches people have done online have been used in successful prosecutions.

Some people may not want information about their searches shared

But in the UK and US there are laws in place that limit how much and what types of information can be requested. So-called "fishing expeditions" in which police forces or intelligence works request data and look through it for people that have committed crimes are outlawed.

Danny Sullivan, net consultant and founder of Search Engine Watch, said the request Google is fighting was not tied to any criminal investigation by law enforcement organisations.

"This is a weird one and is not something that has come up before," he told the BBC News website.

He said the fear was that the US Government wanted to set a precedent with this request so it can turn to search firms whenever they want, for whatever data is deemed important.

Certainly, he said, the data being requested is innocuous and has little in it that could tie it to individuals.

But, said Mr Sullivan, because the potential is there to link people with searches for particular terms, such as bomb making materials, the worry is that governments will use the data to monitor and spy on civilians.

Increasingly, he said, search firms were working to personalise their service and it is possible to tie some searches to particular users or machines.

Information online

It is clear that the more that people do via the net, the greater the chance to keep an eye on them. And it is not just data from Google that can be useful.

In early January talented hacker Tom Owad showed what could be done with Amazon's wish list to uncover "subversives".

Mr Owad downloaded the five gigabytes of raw data that makes up the wishlists of the 260,000 Amazon customers that share a common male name.

Writing about his work, Mr Owad said it would be easy to use this data to find out which people are looking for banned books or works considered to be dangerous.

Writing about the row, net analyst John Battelle said net users were only just beginning to come to terms with the trust they transfer to firms such as Google, Yahoo and others to properly safeguard often deeply personal information.

"And I'm not sure either we or they are entirely sure what to do with the implications of such a transfer," he wrote. "Just thinking about these implications makes a reasonable person's head hurt."