DENVER -- The FBI recorded the license plate numbers of a dozen vehicles spotted "in the vicinity of" a Denver bookstore where anti-war demonstrators gathered before a rally, according to an agency document released by the American Civil Liberties Union Tuesday.

"This report raises more questions about the degree to which the FBI is unjustifiably regarding demonstrations and public dissent as potential terrorism," said Mark Silverstein, legal director of the ACLU in Colorado. "Why is the FBI conducting surveillance of a bookstore, monitoring the persons who gather there, and keeping files with lists of license plate numbers?"

The ACLU obtained the FBI report under the Freedom of Information Act.

The ACLU said the surveillance was part of a domestic terrorism investigation by the FBI's Joint Terrorism Task Force. A copy of the document posted on an ACLU Web site does not mention the task force or domestic terrorism.

FBI spokesman Bill Carter in Washington said he was not familiar with the bookstore surveillance, but he said the agency would not monitor a group simply because it is protesting.

"Our interest is not in the First Amendment activities the group is involved in," he said. "It's only when those individuals that may be in that meeting who may be involved in planning, or are actively involved in, violent criminal activity."

The document says a two-hour surveillance of the bookstore showed that at least 40 people "appeared to be involved" in the Revolutionary Anti-War Response demonstration planned later that day in Colorado Springs.

It does not say what led to that conclusion, but it says some of the group "wore all black clothing, including sweat shirts or jackets with hoods." It says FBI agents saw several pink and black flags and banners but doesn't list what the banners said.

The document gives general descriptions of the 12 vehicles but their license numbers are blocked out.

The names of four FBI agents who conducted the investigation are also blocked out.

The report released Tuesday is available on the ACLU of Colorado’s web site.