The FBI has used at least 13 shell companies to conceal a fleet of surveillance planes that recently flew over more than 30 cities in 11 states, according to an Associated Press investigation published today.

According to the AP's report, the low-flying planes are equipped with video, and operate without a judge's warrant — although the government says they're for specific investigations. The AP traced the companies — with names like FVX Research and KQM Aviation — that were used to keep the fleet of at least 50 FBI planes under wraps. Many of the companies' documents were signed by a "Robert Lindley," but the government would not confirm to the AP whether that was a government employee.

In November, The Wall Street Journal revealed that the Justice Department had been equipping planes with so-called "dirtboxes," machines that could scoop up data on cellphones below, which the AP confirms in its report. But the scope and limits of the project are still obscure.

The FBI, according to the AP, asked that the publication's report not be published because it "would saddle taxpayers with the expense of creating new cover companies to shield the government's involvement, and could endanger the planes and integrity of the surveillance missions."