A serviceman has died during a training exercise involving around 20 personnel at an RAF weapons range.
Emergency services were called to RAF Tain, north of Inverness, at around 6pm on Tuesday.
Police Scotland said a firearm was involved in the death of a member of the Armed Forces and that there were no other casualties.
Chief Inspector Iain MacLelland said around 30 detectives are involved in the investigation.
He said: "Police Scotland can confirm that just before 6pm last night, we received a call from the Scottish Ambulance Service reporting an incident at the RAF Tain firing range involving members from the Armed Forces who were on a training exercise.
"Police along with the Scottish Ambulance Service attended there, where one person was found to be fatally injured.
"There were no other casualties as a result of this incident and their next of kin have been informed.
"Both the police and the military are providing them with support at this time and our sympathies are obviously with them in what must be an extremely difficult time for the family.
"Police Scotland are leading the investigation to establish the full set of circumstances of this incident and we are working closely with the Crown Office and Procurator Fiscal Service, the Health and Safety Executive, and the Armed Forces.
"I would wish to emphasise that this incident was confined to the RAF firing range at Tain, that no persons were involved other than those military personnel participating in the exercise, and at no stage was there a threat to the wider public safety.
"The nature of this incident will require a continued police presence in the area over the days ahead and we would ask for the local community's co-operation and we will continue to keep them updated."
MacLelland said the victim was not from the local area.
He added: "We are keeping a very open mind into the cause of the incident. It is the very early stages of the investigation and as the investigation progresses we'll continue to release further information into the nature of that inquiry."
Scotland's First Minister Nicola Sturgeon said: "This is a tragic incident and my deepest condolences go to the family, friends and colleagues of the person who has died.
"It is another important reminder of the often difficult and dangerous job our Armed Forces do every day."
A cordon was set up around the base on the Dornoch Firth, in Easter Ross, while authorities dealt with the incident.
The training facility is used by the RAF and the Army. It has a rifle range and a small arms range as well as bombing ranges for aircraft.
The range is also used by US and other Nato air forces for bombing and strafing practice. It has 18 targets suitable for a variety of weapons.
The death comes a little more than two months after a soldier was shot dead during a live-firing exercise at Otterburn Training Area in Northumberland.
Private Conor McPherson, 24, a member of The Black Watch, 3rd Battalion, the Royal Regiment of Scotland, was killed on August 22.
Just over a month before that, a soldier died on a training exercise on the hottest day of the year in Brecon, South Wales.
Joshua Hoole, from Ecclefechan, near Lockerbie, Dumfries and Galloway, was on pre-course training for the Platoon Sergeants' Battle Course when he died on July 19.
The 26-year-old member of The Rifles regiment collapsed after a fitness test near the Dering Lines infantry training centre at about 6.30am.
Police are investigating the circumstances surrounding both earlier deaths.