(2:05 p.m. EDT) -- Insufficient oil pressure was the "direct cause" of the engine failure that left Viking Ocean Cruises' Viking Sky without power on March 23, according to Norwegian Maritime Authority (NMA) officials.

Viking Sky suffered an engine failure that left it adrift in rough seas, more than two miles away from the Norwegian coast. After sending out a mayday call, the ship's crew evacuated 479 passengers by helicopter, before regaining power and returning to shore under tow assistance with the remaining passengers and crew.

A March 27 press release issued by the NMA states: "Throughout the night, the NMA has worked together with the ship's classification society, Lloyd's, and the company in order to identify the reason why the Viking Sky suffered power 'blackout' at Hustadvika in challenging weather conditions on Saturday 23 March.

"For the present, our conclusion is that the engine failure was directly caused by low oil pressure."

The NMA went on to say the level of lubricating oil in Viking Sky's tanks was within set limits, but was relatively low when the ship began to cross Hustadvika. Although the tanks were equipped with level alarms, they did not sound until the ship was already in rough seas.

"The heavy seas in Hustadvika probably caused movements in the tanks so large that the supply to the lubricating oil pumps stopped," the release continued. "This triggered an alarm indicating a low level of lubrication oil, which in turn shortly thereafter caused an automatic shutdown of the engines."

As a result of the incident, the NMA has issued a general safety notice about the need for continuous oil lubrication to engines and other critical systems during poor weather conditions.

Viking responded with its own statement: "We welcome the prompt and efficient investigation carried out by the NMA and we fully understand and acknowledge their findings.

"We have inspected the levels on all our sister ships and are now revising our procedures to ensure that this issue could not be repeated. We will continue to work with our partners and the regulatory bodies in supporting them with the ongoing investigations."

Engine failures are unusual in the cruise industry, but evacuations are extremely rare. Factors that can lead to engine failures include mechanical issues, human error or improper maintenance.

Regardless of the cause, every cruise line has a procedure in place for responding to engine failures, with the safety of all passengers and crew being the first priority. Passengers can also be proactive about cruise ship safety in their own ways, by paying attention during the ship's mandatory safety exercise (muster drill). 

Read accounts from Cruise Critic members who were onboard Viking Sky's evacuated sailing, to learn more about how passengers reacted to the experience, and how the crew responded.