A grisly scene unfolded off the coast of Vancouver Island, Canada when a male orca attacked and killed a newborn calf—with help from his mother. As Brandon Specktor reports for Live Science, this is the first recorded case of infanticide among orcas, and the only time that a non-human mother and son have been seen working together to kill an infant.

The incident took place in 2016 and was observed by marine biologists at the research station OrcaLab, who recently described the attack in the journal Scientific Reports. On that fateful day, scientists picked up strange calls on an underwater microphone and set out to investigate. They came across a 28-year-old mother traveling with several of her young, including an infant that was likely only a day old, judging by its visible fetal folds (only apparent a few days after birth) and a dorsal fin that was not yet erect. Lurking behind the family was a 32-year-old male and his 46-year-old mother.

Suddenly, the researchers write in the paper, they saw “erratic movements and splashing suggestive of a predation event.” They also noticed that the baby orca was not surfacing with its mother. When the adult male swam past the researchers’ boat, they could see that he had the calf’s fluke clamped in its mouth, the newborn’s body trailing beneath his jaw.

“We were a bit horrified, but more so I think we were fascinated,” Jared Towers, a cetacean researcher with Fisheries and Oceans Canada, tells Amy B. Wang of the Washington Post. “We knew that it was time to just collect as much data as we could to accurately record our observations.”

The researchers watched as the mother orca tried desperately to save her baby. At one point, she rammed the male so hard that his body undulated and a spray of blood and water flew into the air. But the mother of the male aggressor intervened in the fight, blocking the newborn’s mom from attacking her son. According to a video published on the Facebook page of the BC Killer Whale Research Report, the male and his mother spent the next four hours taking turns pushing and dragging the baby around. Unable to surface for air, the infant ultimately drowned.

Terrestrial species like primates and rodents have been known to commit infanticide, and the behavior has been infrequently observed among dolphins. Scientists have suggested a number of reasons why animals might kill infants of their own species—one of which is cannibalism. But the OrcaLab researchers did not see any signs of feeding, leading them to suspect that the infant was not killed as prey.

It is more likely, they suggest in the paper, that the male was trying to mate with the mother of the newborn calf. As Sarah Gibbens explains in National Geographic, female orcas are unable to breed while they are nursing their offspring. So once the mother stopped lactating, she would have been ready to mate once again.

But why did the male’s mother get involved in the killing? The researchers speculate that she may have been trying to ensure the success of her lineage by helping her son mate. Female orcas and their children can share strong bonds, and have been known to cooperate in predation. In this case, it is possible that mother and son were collaborating to remove an obstacle—the baby orca—that was preventing them from passing on their genes.

“I think we don’t give a lot of animals enough credit for their ability to plan and think ahead,” Towers said in an interview with Gibbens of National Geographic. “[B]ut I think that’s exactly what was happening here.”