The exact day the film was made is unclear, but Baltimore County police confirmed the department is investigating after the video was posted July 18 on YouTube from the Druid Ridge Cemetery under the user name "BrownsFan4Life."
The video, which has since been taken down, is posted on other sites, such as Deadspin.
In the video, the cameraman focuses on what seems to be a Ravens fan wearing an Ed Reed jersey standing at Modell's grave.
The man then tears that jersey off to show a Browns jersey, at which point he relieves himself on Modell's grave, saying he had "no choice" because Modell "tried to kill" the Browns when he moved the original team to Baltimore 18 years ago.
The fan makes no attempt to hide his face. What's more, he displays his identifiable tattoos in slow-motion sequence at the end.
Modell died in 2012.
The Browns returned to Cleveland as an expansion franchise in 1999.
Baltimore police said the investigation is ongoing.