A fiery collision between to oil supertankers off Singapore took a bizarre turn Friday, after the crew of one of the vessels turned off its onboard tracking system and fled the scene, according to news reports.

The Malaysian Coast Guard caught up with the São Tomé and Príncipe-flagged tanker Ceres I after it tried to use two tugboats to make its escape, Splash247 reports.

The dramatic crash raised “fresh questions over the rising risks to maritime traffic, the environment, and human lives in one of the world’s busiest waterways,” Bloomberg writes.

“It wasn’t immediately clear what caused the collision, though the aging Ceres I, empty on its way back from China, underlines the challenges that come with more older, often uninsured vessels moving sanctioned [prohibited] crude through the Straits of Malacca,” the news story adds. “The waters to the east of the Malaysian peninsula have long been a hub of dark fleet activity.”

The Ceres I, which was empty at the time but still caught fire, collided with the Singapore-flagged Hafnia Nile, a Panamax tanker with capacity for up to 500,000 barrels of oil. News reports had the Hafnia Nile running full with 300,000 barrels of naphtha, a petrochemical feedstock derived from crude oil.

Authorities evacuated 22 crew from the Hafnia Nile and 14 from the Ceres I, and the 26 crew members who stayed aboard the Ceres I to fight the fire were reported safe, according to shipping industry media. Malaysia reported a 17-square-kilometre oil slick in the area, Splash247 says.

While the Hafnia Nile was built in 2017, the Ceres I went into service in 2001, “meaning it’s still in use well beyond when most owners would consider scrapping a vessel,” Bloomberg states. Its property and indemnity (P&I) insurer is unknown, “a characteristic common to almost all dark fleet ships,” the news agency adds.

Bloomberg says industry intelligence shows the Ceres I carrying cargo from Iran to Venezuela last year, and industry media say the vessel “discharged Iranian crude via ship-to-ship transfers in Malaysia’s Linggi transshipment hub” as recently as April.

“Dark fleet vessels have caused problems for Singapore before,” Bloomberg adds. “A shadow fleet oil tanker ran aground near Singapore in December, and earlier last year a ship called the Pablo exploded near Malaysia. Its burning wreckage sat for months off the country’s coastline.”