The New York Police Department has begun policing how its officers use Facebook, Twitter and other social media. An internal order made public on Thursday advises members of the nation's largest police department to be careful with what they reveal online—even urging them not to disclose that they're on the force. Officers "are to exercise good judgment and demonstrate professionalism expected of them while performing their official duties," the memo says. It also warns that "personal social media sites may be used against them to undermine the credibility of the department, interfere with official police business, compromise ongoing investigations and affect their employment status." The guidelines bar officers from posting photos of themselves in uniform—with the exception of those taken at promotion or awards ceremonies—unless they have permission from the department. Officers could face discipline if they don't comply. Police officials said the policy has been in the works for about two years, and arose out of concern that police officers' online postings could embarrass the NYPD or be misinterpreted as official police policy. The department punished more than a dozen officers after they made degrading remarks about revelers at the West Indian Day Parade in 2011. "We believe these guidelines are reasonable and make sense," Police Commissioner Raymond Kelly said Thursday. The Patrolmen's Benevolent Association, which represents 23,000 police officers, declined comment. In the past, the union has cautioned its members about what they post and who they interact with on the Internet. The NYPD edict prohibits the posting on personal websites of crime scene photos or witness statements. It also bars officers from using social media to contact witnesses, crime victims or lawyers involved in pending cases, or to contact minors who aren't part of their families. "Such communications may be deemed inappropriate or unethical and may jeopardize an ongoing investigation," it says. The adoption of guidelines was first reported in the Daily News .