The Department of Homeland Security (DHS) has monitored the Twitter account of prominent civil rights activist DeRay Mckesson, referring to him as a "professional protester" who is "known to law enforcement," according to documents obtained by VICE News in response to a Freedom of Information Act (FOIA) request.

Mckesson's Twitter and other social media accounts were being monitored by DHS last May during the height of the protests in Baltimore that followed the death of Freddie Gray, a 25-year-old black man who was critically injured while in police custody. DHS took note when McKesson, a former Minneapolis public school official and an activist in the Black Lives Matter movement, posted details to his Twitter account about a planned protest in Baltimore.

One email said DHS "social media monitors have reported that a professional demonstrator/protester known to law enforcement (Deray Mckesson) has post on his social media account that there is going to be a 3:00 pm rally at the FOP#3 lodge located @ 3920 Baltimore Ave, Baltimore, MD 21211 … This is early raw unevaluated and uncorroborated reporting at this time."

The subject line of the email was "FYSA," which is short for "For Your Situational Awareness." Kade Crockford, the director of the American Civil Liberties Union's Technology for Liberty project, told VICE News that the term "situational awareness" is a cover that allows police departments and federal law enforcement agencies such as DHS to conduct "surveillance of lawful, First Amendment-protected political speech," which includes tweets.

"Those two words don't change what police are doing, however: spending precious public resources watching activists' internet use, and sending out reports to other law enforcement officials tracking the protected political activity of law abiding dissidents," Crockford said. "Apologists for this kind of chilling, wasteful government spying would likely say that Deray Mckesson's tweets are public, and therefore he has no right to privacy in them. But just because police can do something doesn't mean they should."

Mckesson, who is at a protest today in St. Louis, did not respond to a request for comment from VICE News.

Mckesson boasts more than 200,000 followers on Twitter, and uses social media to keep the public informed about the Black Lives Matter movement. Teach for America, which gave Mckesson the Peter Jennings Award for Civic Leadership last month, said his "deft use of social media provided Ferguson citizens with a viral voice."

Crockford pointed out that Mckesson was arrested on Monday in St. Louis by an official "wearing an overly militarized 'Department of Homeland Security' outfit," simply because he was "just standing and asking questions, committing no crimes."

Mckesson was in St. Louis participating in protests marking the one-year anniversary of the death of teenager Michael Brown, who was killed by white police officer Darren Wilson in Ferguson, Missouri. Mckesson was arrested during a sit-in at the federal courthouse in downtown St. Louis, and was issued a summons for allegedly obstructing the courthouse doorways.

"In light of this [email] exchange including a DHS employee, we must ask if DHS officials knew of [Mckesson's] presence beforehand and planned to arrest him on suspicion of committing the grave future crime of active democratic participation," Crockford said. "How long has DHS been watching McKesson? For what purpose, exactly? Nothing about this behavior 'secures' the "homeland' — to the contrary, attacks like these on Mckesson and the Black Lives Matter movement's civil rights and civil liberties undermine the ideals at the very foundation of a free society."

DHS spokesman Sy Lee told VICE News the department's National Operations Center "employs social media monitoring for situational awareness purposes, within the clearly defined parameters articulated in our Privacy Impact Assessment, to ensure that critical information reaches appropriate decision-makers in federal, state, local, tribal and territorial governments."

Lee did not respond to questions about why DHS characterized Mckesson in the email as a "professional demonstrator/protester."

According to copies of DHS's media monitoring manuals obtained by VICE News, the National Operations Center (NOC) Watch, also referred to as "the Watch," "provides critical functions and capabilities that are central to implementing responsibilities of the Department of Homeland Security (DHS)."

Thousands of pages of documents obtained by VICE News from the FBI and DHS over the past year make clear that the agencies have been closely following social media and using those platforms to gather intelligence on protest movements and protest leaders. It has become increasingly clear, the government documents show, that federal law enforcement also views high-profile protests, such as the one in Baltimore, as a possible breeding ground for domestic terrorist activity, and a way to cultivate informants.

DHS and other agencies have explained that the presence of federal law enforcement officials in Baltimore, St. Louis, and elsewhere is to protect federal property, such as courthouses, from possible damage by protesters.

Aside from the email about Mckesson, the rest of the documents in the Baltimore trove turned over to VICE News show that DHS constructed a "geospatial" map to follow the protests, that officials were concerned about potential "anarchist violence" in the area, and that they tried to confirm rumors that the singer Rihanna was going to perform in the area. (Emails obtained by the Baltimore Sun suggested that Baltimore police blocked the concert.)

One email says budget approval was needed to deploy dozens of DHS officers to the area, at an estimated to cost taxpayers of $60,000, for "Operation Straw Temple." A DHS timeline of the protests say Baltimore Police Department requested assistance from law enforcement officers working for DHS's Federal Protective Service (FPS) to protect two fire stations near federal property." Additionally, the timeline says an "FPS marked police vehicle was vandalized with a brick being thrown into the rear window at [law enforcement officer's] residence."

The documents reveal that DHS was also gleaning intelligence about activities in Baltimore from the Twitter accounts of other individuals and groups.