CHICAGO — Private Facebook groups have "emboldened" sellers of illegal drugs and guns to connect with potential buyers over the social media site, Chicago police said Tuesday, as leaders announced that a two-year undercover investigation led to more than 50 arrests.

Police leaders, including Chicago's new interim superintendent, also accused Facebook of failing to help prevent illegal sales of guns. The social media company banned private sales, trades and exchanges of firearms in 2016, but investigators said they found dealers using private groups and messages to quickly sell firearms and drugs at prices higher than street values.

First Deputy Superintendent Anthony Riccio said Facebook agreed to shut down groups identified during the Chicago investigation but that it also should kick members of those groups off the site.

"Facebook often cites privacy concerns when they are confronted with the facts of our investigation," Riccio said. "The truth is, Facebook is harboring criminals. These criminals know how to use the privacy Facebook affords them and they profit from the sales of illegal drugs and dangerous guns."

Riccio also said police have been frustrated by Facebook's removal of fake profiles that investigators use to pose as potential buyers.

Facebook spokeswoman Sarah Pollack said the company quickly responds to "valid legal" requests from police.

"Illicit drug and firearms sales have no place on our platform," Pollack said. "We remove content and accounts that violate our policy and catch over 97% of drug sale content and over 93% of the firearms sales content we remove before it is reported to us."

The company's instructions for law enforcement say a subpoena is required to share a subscriber's records including name, email addresses and location information on recent log-ins; disclosing contents of an account requires a federal or state search warrant. The site also says all Facebook users must use "the name they go by in everyday life," and fake accounts will be penalized.

Facebook says it uses detection technology to find content that violates its policies banning the sale of drugs or firearms, including posts in private groups.

Chicago police leaders have blasted Facebook after previous investigations of illegal guns and drug sales on the site. In 2017, then-Superintendent Eddie Johnson said the company was failing to cooperate with police cracking down on the activity.

Tension over law enforcement's use of social media networks exists in other areas too; for example, police in Memphis were sued by the state's branch of the American Civil Liberties Union in 2018 for using an undercover Facebook account to monitor protest groups' activities.

Personal privacy advocates say Facebook could do more to protect users from that type of police activity and keep meeting its baseline responsibility to hold law enforcement to the same rules as everyone else on the platform.

"Police shouldn't get to follow different rules than members of the public," said Dave Maass, a researcher for the Electronic Frontier Foundation. "They may say 'Oh, this is to cut down on gun sales.' The next thing you know, you're searching social media for information on First Amendment activities or whether they've been driving while texting."

Charlie Beck, Chicago's interim police superintendent and the former head of the Los Angeles Police Department, said Tuesday that Facebook users' privacy rights don't "trump the rights of the general public."

"Another person's rights have to stop where the safety of another individual becomes in jeopardy," Beck said. "That's what laws are all about."