Over the last two months, scientists have been struggling to figure out why orcas have been attacking sailboats along the Spanish and Portuguese coasts.

The most recent attack occurred on Friday off A Coruña, on the northern coast of Spain, The Guardian reports. Pete Green, the managing director at Halcyon Yachts, revealed the company was transporting a 36-foot boat to the U.K. when an orca rammed into its stern at least 15 times. The boat lost steering and had to be towed to a port.

READ: ‘Dolphin Stampede’ Caught On Video Shows Over 300 Mammals Porpoising

Researchers, who study a small population of the intelligent mammals in the Strait of Gibraltar, find the recent attacks quite odd. While orcas typically follow boats closely out of curiosity, ramming a vessel is out of character.

Victoria Morris was on a 46-foot boat in Spain on July 29 when an orca attacked the vessel. The attack lasted over an hour, and Morris feared she would have to abandon ship.

“The noise was really scary. They were ramming the keel, there was this horrible echo, I thought they could capsize the boat. And this deafening noise as they communicated, whistling to each other. It was so loud that we had to shout,” she said.

Following the report, in which Morris suggested that it felt as if the attack was “totally orchestrated,” scientists stated the behavior was “highly unusual” and “concerning.”

Rocío Espada, who works with the University of Seville’s marine biology laboratory, revealed she has never seen this type of behavior from orcas. “For killer whales to take out a piece of a fiberglass rudder is crazy,” she explained.

“I’ve seen these orcas grow from babies, I know their life stories, I’ve never seen or heard of attacks,” Espada added.

Scientists believe it’s too early to tell why orcas are attacking sailboats, but some have suggested the mammals may be reacting to stress.

Orca Killer Whale
An orca swims with its calf at the Marineland animal exhibition park in the French Riviera city of Antibes, Dec. 12, 2013. Valery Hache/AFP/Getty Images