The FBI has told Congress not to worry about its shell-company-owned surveillance aircraft, which are decked out in the best surveillance tech, as they are engaged in an unclassified operation - which they were unwilling to talk about in a Congressional briefing.

The snooping Cessnas were spotted over Baltimore during the protests and riots in the city after the death of Freddie Gray.

An Associated Press investigation confirmed the normality of such missions, and revealed more than 100 flights over cities including Houston, Phoenix, Seattle, Chicago, Boston, Minneapolis, as well as Southern California.

AP has now reported more information about the Feds' activities following a Capitol Hill briefing on Wednesday, which the newswire reckons may be the first ever effort "to impose oversight for the FBI's 30-year aerial surveillance programme."

The FBI representatives declined to answer some questions regarding its programme during an unclassified portion of the briefing, despite having described it as unclassified and not secret. The FBI was especially sensitive to mentioning the number of planes it flies and the cost of the programme.

AP notes that in a 2009 budget document, the FBI declared it had 115 planes in its fleet.

"The ubiquity of the flights, combined with few details about the surveillance equipment aboard the planes, raised civil liberties concerns over Americans' privacy," said the newswire.

The planes had, on rare occasions, been fitted with what were likely IMSI catchers – technology which has been criticised for its inability to target mobile phones for surveillance, instead sucking up thousands of local communications.

This technology had only been used on the surveillance aircraft five times since 2010, the FBI told the hearing, but the g-men refused to disclose how often it had used that technology in ground operations, according to a Senate staffer who spoke to AP on the condition of anonymity.

Two senators – Dean Heller, a Nevada Republican, and Ron Wyden, an Oregon Democrat – have introduced a bill to limit aerial surveillance without a warrant.

"Technology has made it possible to conduct round-the-clock aerial surveillance. The law needs to keep up," Wyden said in a statement. "Clear rules for when and how the Federal government can watch Americans from the sky will provide critical certainty for the government, and help the unmanned aircraft industry reach its potential as an economic powerhouse in Oregon and the United States."

AP reports that the FBI said it does not comment on pending legislation, but maintained that a warrant was not necessary for the type of surveillance being conducted from its planes.

Explaining how they managed to expose the surveillance planes as belonging to the FBI, the journalists said they had noted one of its shell companies shared a post office box with the Justice Department, which created "a link between the companies and the FBI through publicly available Federal Aviation Administration records".

AP reports that the FBI told Senate staffers it was working with the FAA to restore some cover to preserve operational security, but it did not plan to spend the money required to operate under "deep cover". ®