Police are investigating a video linked to a man who was found with a crossbow at Windsor Castle on Christmas Day.
The footage, obtained by The Sun newspaper, appears to show a masked person in a hoodie holding a crossbow.
They are shown addressing the camera saying they wanted to "assassinate the Queen" in a "revenge" mission.
Scotland Yard confirmed detectives were "assessing the contents of a video" following the arrest of a 19-year-old man from Southampton.
The police spokesman declined to comment on the identity of the person in the video.
Buckingham Palace has also declined to comment.
A 19-year-old suspect was sectioned under the Mental Health Act after being found in the castle grounds.
He was arrested at about 08:30 GMT on Christmas Day and was initially held on suspicion of breach or trespass of a protected site, and possession of an offensive weapon.
The Queen was in residence at Windsor Castle for Christmas and was due to be joined for lunch by the Prince of Wales and the Duchess of Cornwall, as well as the Earl and Countess of Wessex.
They are shown addressing the camera saying they wanted to "assassinate the Queen" in a "revenge" mission.
Scotland Yard confirmed detectives were "assessing the contents of a video" following the arrest of a 19-year-old man from Southampton.
The police spokesman declined to comment on the identity of the person in the video.
Buckingham Palace has also declined to comment.
A 19-year-old suspect was sectioned under the Mental Health Act after being found in the castle grounds.
He was arrested at about 08:30 GMT on Christmas Day and was initially held on suspicion of breach or trespass of a protected site, and possession of an offensive weapon.
The Queen was in residence at Windsor Castle for Christmas and was due to be joined for lunch by the Prince of Wales and the Duchess of Cornwall, as well as the Earl and Countess of Wessex.