More than 1.5 billion government and private sector records about US citizens and foreigners are stored in an FBI database, declassified documents have revealed.
The database is kept at the FBI's National Security Branch Analysis Center (NSAC) near Washington, according to the documents acquired under a freedom of information request by US magazine Wired.
Data has been drawn from a wide variety of sources, including records of international travel, hotel bookings, car rentals, department store transactions and active aircraft pilots.
The FBI data mining system brings the US government closer than ever to implementing the total information awareness system proposed by the Pentagon after the September 11 attacks, reports said.
Such a system would be designed to correlate data from a large number of different sources to automatically identify potential terrorists and other threats.
But the proposal has been criticised by privacy groups as ineffective and invasive. They say the declassified documents show that the government is going ahead with the plan in secret and without strict independent supervision by public bodies.
Critics have expressed concern that the US government is using an unproven technology that could give false positives that would subject innocent people to unnecessary scrutiny.
Strict enforcement of data protection legislation is the only way to guard against secret FBI-style databases of personal information, says a UK-based security expert.
Peter Sommer, professor of security at the London School of Economics, said huge amounts of information are collected by commercial organisations as part of 'legitimate customer profiling'.
"The problem is that it is only rigorous enforcement of data protection legislation which prevents them being used to form a large data mining operation by intelligence agencies," he said.
Sommer said this was the reason for that it is important for privacy watchdogs like the UK's Information Commissioner's Office (ICO) to have sufficient powers and resources.
According to data mining firm Detica, most people do not realise how many organisations are collecting their "digital footprints" as they roam the internet and communicate with their friends.
Anthony Golledge, head of Detica's technical consulting practice said that technology was changing so fast that most people were not aware of how many "digital footprints" they leave as they surf the internet, travel and use their mobile phones.