JOHANNESBURG (AP) — The oil spill disaster off Mauritius turned deadly this week when a tugboat leaving the shipwreck collided with a barge and sank, killing at least three sailors, police said Tuesday.

The tug was towing the empty barge from the stranded hull of the Japanese ship, the MV Wakashio, on Monday night when heavy seas rammed the barge into the tug. It sank off the northeast coast of the Indian Ocean island nation, near a village called Poudre D'or.

Four sailors were rescued and one is missing, said Mauritius police constable Jordan Jason Sunasy.

“It is a terrible twist in this disaster," environmental consultant Sunil Dowarkasing told The Associated Press. “The consequences of the shipwreck has now taken human lives. The cost of the oil spill keeps mounting.”

The tugboat sank in deep water outside a coral reef and so far no oil has been seen on the coast, Dowarkasing said.

The environmental disaster began on July 25 when the bulk carrier Wakashio strayed miles off course and struck a coral reef a mile offshore. After being pounded by heavy surf for nearly two weeks, the ship's hull cracked and on Aug. 6 it began leaking fuel into the Mahebourg Lagoon, polluting a protected wetlands area and a small island that was a bird and wildlife sanctuary.

More than 1,000 tons of fuel spilled into the coastal waters. About 3,000 tons remained on the boat and was pumped into barges before the Wakashio broke in two several days later.

Thousands of civilian volunteers worked for days to try to minimize the damage, creating makeshift oil barriers by stuffing fabric tubes with sugar cane leaves and using empty plastic bottles to keep them afloat. Environmental workers carefully ferried dozens of baby tortoises and rare plants to shore, plucking some trapped seabirds out of the goo.

Dead dolphins began washing up on the coast last week. So far 47 dolphins and three whales have been found, Dowarkasing said. An initial autopsy by a government laboratory said oil was not the cause of the deaths, but few Mauritians believe that, Dowarkasing said.

The government of Prime Minister Pravind Jugnauth is under pressure to explain why immediate action was not taken when the ship first ran aground. He has blamed bad weather for the government’s apparent inaction.

Tens of thousands of people marched in the capital on Saturday in protest, calling on top officials to step down.

The ship's captain and first officer have been arrested and charged with “endangering safe navigation,” and several investigations are underway./