Intruders broke into the grounds of Windsor Castle last month and stole two vehicles, the police confirmed on Monday, a significant security breach at the sprawling royal estate west of London that is the primary residence of Prince William, his wife Catherine, and their three children.

William, the heir to the throne, and his family are believed to have been at home at the time of the burglary, according to The Sun, a London tabloid, which first reported the incident on Sunday. King Charles III and Queen Camilla also stay often at Windsor Castle, but were not there at the time.

“Offenders entered a farm building and made off with a black Isuzu pickup and a red quad bike,” the police spokesman said. “No arrests have been made at this stage and an investigation is ongoing.”