A pilot is among three suspects being investigated following the collision between a Tsakos tanker and a Norwegian frigate last year.
Police told NRK that the pilot onboard the 113,000-dwt tanker Sola TS (built 2013), an officer on the frigate Helge Ingstad and an employee of the local vessel traffic services are being investigated.
The November incident led to the warship sinking.
District prosecutor Ole Bjorn Mevatne said the pilot is suspected of breaching navigation rules.
The frigate's duty chief is also suspected of violations of navigation rules, as well as negligence.
None has been charged.
The investigation is expected to continue until the summer.
A preliminary investigation in November found the crew of Helge Ingstad did not realise they were headed toward a moving tanker until it was too late.
The Accident Investigation Board Norway said the crew of the state-of-the-art naval ship, having just completed a watch change while returning from Nato exercises in the early hours of 8 November, thought Tsakos Energy Navigation's ship was stationary until the two were within 400 metres of each other.
The frigate's crew then attempted an avoidance manoeuvre, but it was too late.
