Japan's transport safety authority concluded Thursday a cargo ship ran aground and spilled oil off Mauritius in 2020 because it altered its planned route and approached the coast in order to pick up mobile phone signals.

The Japan Transport Safety Board also said that COVID-19 travel restrictions and border closures may have slowed containment efforts and increased the damage caused by the massive oil spill.

The bulk carrier Wakashio, chartered by Mitsui O.S.K. Lines Ltd. and owned by Nagashiki Shipping Co. in Okayama Prefecture, western Japan, spilled about 1,000 tons of fuel oil into a pristine marine environment in the Indian Ocean.

The ship with 20 people aboard was heading to a Brazilian port. No one died in the accident off the southeastern coast of Mauritius.

The board's three-year investigation also found that safety awareness among crew members was low as the freighter had repeatedly approached close to coastlines to connect to mobile phone networks in the past as well.

Photo taken on Aug. 15, 2020, shows stranded Japanese cargo ship Wakashio, broken into two, off Mauritius. (Photo courtesy of the Japan Transport Safety Board)(Kyodo)

According to the report, the captain of the 101,932-ton vessel ordered it to divert from its planned route and approach the coast without obtaining marine charts of the area, leading to the grounding of the ship on the evening of July 25, 2020.

Marine charts would have alerted the ship to crucial information such as that the water depth along the coastline in the area was shallower than 20 meters.

The route was changed two days before the grounding incident, with the distance from the coast of Mauritius altered from 22 nautical miles to 5 nautical miles.

On the day of the accident, the ship tried to further reduce the distance from the coast from 5 nautical miles to 2 nautical miles, according to the report.

The Indian captain and his first officer, a Sri Lankan national, were focused on signal receptions for their smartphones, the safety board's report said.

The captain also had two glasses of whisky and water at the birthday party of a crew member before the accident.

Removal of the wreck of the Wakashio, which broke into two, by a salvage team was delayed due to weather conditions and travel restrictions associated with COVID-19.

The Japanese government sent disaster relief teams to help with the removal of the oil and assess the environmental damage.

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