The Denver Police Department has begun using social media software that is raising concerns over possible civil rights violations in Denver and other areas of the country where it is used.

Thursday, the ACLU of Colorado questioned the department’s use of the software, saying it could be used to monitor the free speech activities of people who are not suspected of criminal activity. The software’s use also may violate a 2003 agreement — written on the heels of a controversy over police spying on residents — that regulates how and when the department monitors city residents’ activities.

The ACLU of Colorado pointed to an investigation by the ACLU of Northern California that found the Fresno Police Department used search words such as “#BlackLivesMatter” and “PoliceBrutality” when using the software.

“There’s a risk if we know the police department is watching and listening to tweets and posts on certain subjects, people may be afraid to say something,” said Mark Silverstein, legal director of the ACLU of Colorado. “I’m not going to say something about that police shooting because the police might put me in a file. The issue is it has a chilling effect on our First Amendment rights.”

The ACLU filed an open records request Thursday seeking the police department’s contract with Geofeedia, the company that develops and sells the surveillance software. The ACLU also wants to know which officers are authorized to use the software and which search terms they use when conducting surveillance.

The police department’s intelligence bureau in May received $30,000 for a one-year subscription for 30 people to use the software. In a request for the money from the department’s confiscation fund, the intelligence bureau’s lieutenant wrote that it would allow his officers to analyze, predict and act on real-time social media content from anywhere in the world.

The request also cited the Martin Luther King Marade, the annual 420 rally and the Parade of Lights as events where it could be used.

“Geofeedia is also a great investigative tool,” Lt. William Mitchell wrote in the request. “It has the ability to identify criminal suspects and their actions as they post them to social media.”

Mitchell used examples of real events where the software was used, including in Santa Clara, Calif., where police were able to find a woman making threats on social media during Super Bowl 50 festivities and after the Boston Marathon bombing where police used it to help identify potential witnesses.

The Denver Department of Safety issued a statement Thursday that said, “The Denver Police Department utilizes the cloud-based platform to identify open source posts that may assist with the prevention of violent acts and to assist with identifying criminal activity. The platform also allows the department the ability to gain an awareness on events which could have an impact on public safety.”

Efforts to reach Geofeedia company officials were unsuccessful.

Geofeedia allows police to target specific neighborhoods or areas of the city and capture all posts to social media sites, including Facebook, Twitter, Instagram and international social media sites such as the Russia-based VKontakte. Users also can do keyword searches for specific phrases or names, according to Geofeedia promotional material.

In a company video, Geofeedia explains the software by focusing on a demonstration over Israel in Chicago. The user brings up a map of downtown Chicago and draws a circle around the blocks where the march is taking place. The software then identifies all of the social media posts being made about Israel in that area.

The company’s website also features a California wildfire to show how firefighters can use social media to track the fire’s impact.

Geofeedia markets its software to private companies that want to monitor what people are saying about their products and to journalists who can use it to identify trends and stories of interest in their communities. The Denver Post has used Geofeedia to help reporting on floods and Denver Broncos football games, although its account is no longer active.

Civil rights experts across the country have been expressing concerns about potential First Amendment violations connected to Geofeedia and other similar software, said Alan Chen, an expert on constitutional law who is a professor at the University of Denver’s Sturm College of Law.

“The potential for abuse with this software is high,” Chen said. “I don’t have a lot of confidence there will be transparency from law enforcement on how this will be used.”

The ACLU also raised questions about the secrecy surrounding the department’s decision to buy the software.

“There ought to be some opportunity for public input and discussion before the purchase,” Silverstein said.

There also are potential concerns over the software’s implications for the Fourth Amendment, which protects Americans from unlawful searches and seizures.

These types of computerized searches are a growing gray area when it comes to constitutional protections against unlawful searches, said Ian Farrell, an assistant professor at DU’s law school.

In the past, courts have determined there is no expected right to privacy for information shared with third parties whether it be a bank or a partner in crime, Farrell said. So information shared online is open for police to review.

The problem arises in blanket searches of people’s accounts and postings on the internet, he said.

“My personal view is it’s contrary to the spirit of the Fourth Amendment to do this level of mechanized surveillance given the amount of information we routinely make available to people through technology,” Farrell said. “It’s almost impossible to have a normal life in modern times without posting on Facebook, so for the government to expect us to give up privacy to everything we share with friends and family online means very little remains private.”

The ACLU, which learned of the software’s use in Denver through a story on The Daily Dot, finds the software particularly concerning because of a 14-year-old controversy in Denver known as “spy files.”

“There’s a question of what are police doing with this tool,” Silverstein said. “That’s a question that deserves an answer, especially in light of the spy files controversy.”

In 2002, the ACLU helped bring a lawsuit against the Denver Police Department over intelligence files it kept on city residents. The lawsuit claimed the department spied on people who were exercising their First Amendment rights by attending community meetings and protests. Officers conducted surveillance on them even though they had not made threats or otherwise indicated they were a public safety concern.

The police department settled the lawsuit, and, as part of the settlement, it rewrote its policy on intelligence gathering. The policy included directions barring officers from conducting surveillance on a person unless they have a reasonable suspicion to believe the person may be involved in criminal activity.

Collecting social media posts may very well violate that policy, Silverstein said.

“The fear is people will be less willing to attend a rally or participate in a protest if doing so will put them in a police file,” he said. “The social media posts may be public but that’s the same kind of information the police were collecting back in the ‘spy files’ days.”