Boston police are investigating after the sword from the Robert Gould Shaw and Massachusetts 54th Regiment Memorial on Boston Common was found on the ground by a concerned citizen Tuesday.

A police spokeswoman said officials believe the memorial, which is across from the State House, was vandalized.

Sean Hennessey, a spokesman for the National Park Service of Boston, said a park service ranger came to the memorial around 7:30 a.m. after a pedestrian flagged down a police officer to report that the sword was broken off the bronze relief sculpture, created by Augustus Saint-Gaudens.

The sword was given to the Friends of the Public Garden, the group that works in partnership with Boston Parks to tend to the Boston Common, Commonwealth Mall, and the Public Garden, to see if the sword can be reused or if it has to be replaced.

Hennessey said the Friends group works closely with the National Park Service and keeps a supply of replica fiberglass swords on hand, because similar acts of vandalism have occurred in the past.

“It’s fair to say that it has happened with some frequency,’’ he said.

Hennessey said he was unsure whether the latest incident was an act of vandalism, but he doubted “very much that it just fell off.’’

In 2015, a 40-year-old Charlestown man was charged with vandalizing the memorial after he allegedly broke off the sword.

And in 2012, a woman was arrested after police said she threw yellow paint onto the historic monument.

The memorial is a tribute to the 54th Regiment, one of the first all-black units to fight for the Union in the Civil War. It depicts Shaw on horseback, clutching the sword in his hand while marching with soldiers to the battle of Fort Wagner, in South Carolina, on July 18, 1863.

Shaw, who was born into a wealthy Boston family, was felled at the head of his troops and was buried with them in a common grave.

The memorial was unveiled in 1897.

On Feb. 16, Mayor Martin J. Walsh tweeted about the memorial as part of a series of tweets celebrating black leaders for Black History Month.

“Have you noticed the Robert Gould Shaw and the 54th Regiment memorial outside of the State House?’’ he wrote, encouraging residents to learn about the site.

Steve Annear can be reached at steve.annear@globe.com. Follow him on Twitter @steveannear.