The Facebook post included several photos of a smiling Baltimore County police officer, some of him in a suit, another sporting outdoor gear. None showed him in uniform or flashing a badge.

The officer works undercover, and the Facebook poster warned that he investigates gun-related cases. The Facebook user's friend had been arrested in June by the officer in an illegal arms sting. The officer is "known to pose as a gun dealer in order to entrap and arrest people," the post read.

"Please share this."

Law enforcement agencies in Baltimore and across the country are grappling with social media sites and the implications for officers who work undercover, the Baltimore Sun reports. In some cases, publicly identifying an undercover officer on social media has resulted in jail time. Other times, prosecutors say the postings may not constitute a crime, as online speech can be construed as merely sharing — not threatening.l

Such public outings of undercover officers are part of the risks of participating in social media, where there are few rules and filters, prompting some police departments to warn officers to keep a low profile on the Internet.