Federal agencies tried to use vehicle license-plate readers to track the travel patterns of Americans on a much wider scale than previously thought, with new documents showing the technology was proposed for use to monitor public meetings.

The American Civil Liberties Union released more documents this week revealing for the first time the potential scale of a massive database containing the data of millions of drivers, logged from automatic license plate readers around the US.

As President Obama’s nominee for attorney general prepared for a second day of confirmation hearings in Washington, senior lawmakers also called on the US Justice Department to show “greater transparency and oversight”.

Further documents released by the ACLU on Wednesday show that Drug Enforcement Administration (DEA) officials in Phoenix planned on “working closely” with the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to monitor public gun shows with the automatic technology in 2009.

Although the DEA has said the proposal was not acted upon, the revelations raise questions about how much further the secret vehicle surveillance extends, which other federal bodies are involved and which other groups may have been targeted.

“The broad thrust of the DEA is to spread its program broadly and catch data and travel patterns on a massive scale,” Jay Stanley, a senior policy analyst with ACLU, told the Guardian. “This could be a really amazing level of surveillance that we’ve not seen before in this country.”

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