Police are investigating a video in which a masked figure threatens to 'assassinate' the Queen, following a Windsor Castle security breach on Christmas Day

• Police are investigating a video obtained by The Sun newspaper that shows a masked figure threatening to 'assassinate' the Queen.
• It comes after a man was arrested after entering Windsor Castle's grounds with a crossbow on Christmas Day.

Police said Monday they are investigating a video that shows a masked figure saying they want to "assassinate" Queen Elizabeth II, after a man with a crossbow was arrested for entering the grounds of Windsor Castle on Christmas Day.

The video, obtained by the Sun newspaper, showed a person appearing to pose with a crossbow. In it, the person says in a distorted voice: "I will attempt to assassinate Elizabeth, Queen of the Royal Family."

A spokesperson for London's Metropolitan Police told The Washington Post that "following the man's arrest, detectives are assessing the video," but didn't comment on the identity of the person in it.

The Sun linked the video with the intruder. The video was posted on Snapchat minutes before the arrest in Windsor Castle on Saturday morning, The Sun reported. The Metropolitan Police and Buckingham Palace didn't immediately respond to requests for comment from Insider.

Police said a man who was armed with a crossbow was immediately arrested after entering the grounds of Windsor Castle, and did not enter any buildings. The Queen was inside at the time.

"The man was taken into custody and has undergone a mental health assessment — he has since been sectioned under the Mental Health Act and remains in the care of medical professionals," the Metropolitan Police said in a statement.

Being "sectioned" means an individual is detained in hospital after receiving a mental health assessment.

Police did not identify the man, but described him as a "19-year-old man from Southampton."

The Queen was at Windsor Castle for Christmas this year, having cancelled her usual plans to go to her Sandringham estate due to the coronavirus pandemic.

The man was arrested "on suspicion of breach or trespass of a protected site and possession of an offensive weapon," the police said.

Police said an investigation is underway.