This comes amid rising tensions around the country between police and some communities.
The message read - "kill cops" and was signed the vigilante.
It has since been painted over but the Fraternal Order of Police of Prince George's County felt it was important enough to post a photo of it on their Facebook page- urging officers to be extra cautious and watch each other's backs.
"Any statement of that nature needs to be taken very seriously," said John Teletchea, 1st Vice President of the Fraternal Order of Police, Lodge 89. "Hopefully it is a random act of vandalism and not a direct threat to law enforcement in this area."
Prince George's County Police tell us they are aware of the sign and are investigating to see who is responsible - while also remaining vigilant on patrols in the area.
The Police Union says - they hope the public will be vigilant too - and call authorities right away if they hear any talk about harming officers.
This comes amid a growing movement on social media seeking to show support for police this week.
A virginia man created a Facebook event called 'Blue Light Week' to run January 1 thru 7 after seeing the hatred and anger directed at law enforcement officials following the controversial grand jury decisions in Ferguson and New York.
The former police officer and sheriff's deputy is calling on people to put a blue light outside their home to show their support for men and women in blue who are protecting and serving their communities.
Three wives of local police officers are mounting a separate effort on social media to support law enforcement as well.
They are organizing a rally in support of law enforcement officers on January 17th from noon till 3.
The march will start at the National Law Enforcement Officers Memorial and end at the U.S. Capitol they are calling their event - An End The Madness Rally and hoping for a turnout in the thousands.