Four micro-light helicopter pilots are at the centre of an official complaint after allegedly flying through a RAF bombing range in the Highlands.
Tornado fighter jets regularly use the range at Tain
Police are also investigating claims the German tourists flew low over Dunrobin Castle, near Golspie.
The four are alleged to have flown through the RAF range at Tain moments before a Tornado fighter jet performed a practice bombing run last Thursday.
It is understood the Germans were on a flying tour of Scotland.
Tain is regularly used by low-flying fighter jets stationed at RAF Lossiemouth on the Moray coast.
A RAF spokesman said: "We have filed an official complaint through the police and the Civil Aviation Authority about this event.
"It would be inappropriate to go into too much detail at this stage."
Northern Constabulary confirmed it was probing a claim of low flying at nearby Dunrobin Castle.
A police spokesman said: "We are making inquiries into a complaint made by a member of the public on Thursday, 8 June.
"The complaint concerns a group of German tourists and alleged low-flying in their aircraft."
Two years ago a search was launched for a helicopter pilot who flew under the 100ft-high Skye Bridge.
Local people and tourists crossing the bridge watched as an aircraft flew beneath them.
It is illegal to fly within 500ft of the structure.
The year before a pilot avoided prosecution for flying his light plane under the bridge after claiming he only did so to avoid a flock of seagulls.
And around the same time two Spanish pilots flew under the Ballachulish Bridge.