In the wake of last week's riots, federal authorities provided aircraft for surveillance flights over Baltimore, keeping a quiet aerial eye on the lookout for new outbreaks of violence, an FBI spokeswoman said Wednesday.
"The aircraft were specifically used to assist in providing high-altitude observation of potential criminal activity to enable rapid response by police officers on the ground," said spokeswoman Amy Thoreson. "The FBI aircraft were not there to monitor lawfully protected First Amendment activity."
Thoreson did not say what kinds of aircraft were used or who operated them. But the unusual activity in the sky over several days last week has attracted the attention of civil libertarians, who want more details about the aircraft and the flights.
On Wednesday, the American Civil Liberties Union filed Freedom of Information Act requests with the Justice Department and the Federal Aviation Administration seeking more details about flights by two planes.
"These are not your parents' surveillance aircraft," said Jay Stanley, an ACLU analyst. "Today, planes can carry new surveillance technologies, like cellphone trackers and high-resolution cameras that can follow the movements of many people at once.
"These are not the kinds of things that law enforcement should be using in secret."
The trail began with Pete Cimbolic, an aviation enthusiast from Baltimore, who dug through public flight data on the website flightradar24.com and found a small propeller plane flying in tight arcs over West Baltimore last weekend and a jet following a similar path on a larger scale.
The propeller aircraft was a Cessna 182, according to the website.
The Baltimore Sun combed the data on the site and found a third plane over the city last week — a Cessna 206 that made a similar flight April 28, when police and protesters confronted each other on the first night of the citywide curfew.
The plane headed first to the area of Pennsylvania and North avenues, the focus of protests throughout the week, before heading south to Brooklyn when police encountered a group of protesters there.
According to the website, all three aircraft were using identification or "squawk" codes that the FAA says are reserved for law enforcement.
Cessna markets its 182 and 206 models to law enforcement under the brand name Cessna Enforcer.
"You can transform your Cessna propeller aircraft into a stealth eye-in-the-sky machine," its website reads.
The company did not respond to a request for comment.
The airplane manufacturer promotes fixed-wing aircraft as superior in almost every way to the helicopters long used by police. They are stealthier, can fly farther and carry more weight than a helicopter, the company says, while being much cheaper to operate.
Thoreson did not confirm that the three planes were provided by the FBI.
She said the aircraft the FBI provided were used to capture images and help coordinate the police response to unrest in the city.
Plans to fly planes equipped with surveillance equipment have proved controversial elsewhere. In 2013, Dayton, Ohio, backed off a plan to acquire the technology after residents expressed concerns, and a test of a system over Compton, Calif., last year also raised hackles.
Persistent Surveillance Systems was involved in both cases. Ross McNutt, president of the Xenia, Ohio, company, said it also ran a brief operation in Baltimore in 2008.
"We watched ... nearly all of the East and South East Police Districts and witnessed shooting and a range of other crimes during our short operation," McNutt wrote in an email. "We had centered out of the Johns Hopkins medical center and fed the live data into the Police Command Center downtown."
McNutt said his company offers police an effective tool for fighting crime and takes steps to guard the privacy of people on the ground. But the ACLU and other groups worry about persistent and wide-ranging snooping that can capture information about people who are doing nothing wrong.
Civil liberties advocates are particularly concerned that aircraft could offer a powerful platform for controversial new technology that captures information about cellphones.
That tool, known as a "stingray," mimics a cell tower to force all cellphones within range — including those of law-abiding citizens — to connect to it.
Baltimore police recently acknowledged using a stingray thousands of times. The Wall Street Journal has reported that U.S. marshals have a version mounted on aircraft.
There is no indication that the flights over Baltimore were monitoring cellphone information.
While the FBI answered some questions Wednesday, Cimbolic said he plans to keep on digging.
"I'm hoping that I'll be able to figure out a little more with some of the info I already have," he said.