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In 1981, then 17-year-old Christopher Lewis fired a shot at the Queen in her Rolls Royce during a visit to Dunedin but he wasn't charged with treason as police believed he was never close enough to hit her.
A memo, now released publicly by the SIS, confirms that Lewis did intend to kill the Queen.
"Lewis did indeed originally intend to assassinate the Queen, however did not have a suitable vantage point from which to fire, nor a sufficiently high-powered rifle for the range from the target," the SIS memo says.
Witness statements said that a shot was heard nearby, which contradicted initial police statements that reports of a shot being fired were not true.
The SIS documents also show there was no intention of charging Lewis with treason during the on-going police investigation.
Former police officer Tom Lewis, who was originally assigned to Lewis's case, alleges a police cover-up of the incident, including the accused's statements being destroyed.
Police have issued a statement saying they would be overseeing an investigation into the case file of Christopher Lewis.
"Given the interest in this historic matter, the police commissioner Mike Bush has asked the deputy commissioner national operations, Mike Clement, to oversee an examination by current investigation staff of the relevant case file," a police spokeswoman said.
"Given the passage of time, it is anticipated this examination of the old file and its associated material will take some time. NZ Police will share the outcome of this examination once it has been completed."
Lewis, accused of the murder of Auckland woman Tania Furlan, died in Mt Eden prison in 1997.