Researchers completed a necropsy of the cougar that attacked two cyclists in Washington state’s Cascade foothills in May. The report found no signs to explain the animal's attack.

Officials sent the cougar’s carcass to a veterinary lab at Washington State University for a necropsy requesting an examination "for any diseases or abnormalities that may have contributed to its behavior, including rabies testing of the brain,” according to the report.

The report, released Monday by the Washington Department of Fish and Wildlife, said an examination of the cougar showed "no abnormalities."

“An underlying cause that explains the behavior of this cougar was not evident in a complete necropsy examination with extensive ancillary testing," the report said. “Specifically, gross necropsy and histopathology were negative for an underlying disease process likely to cause abnormal behavior or increased aggression (e.g. brain abscess, brain tumor or encephalitis).”

The cougar also tested negative for rabies and any infectious diseases.

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The cougar killed S.J. Brooks, 32, in the attack and seriously injured Isaac Sederbaum, 31, who's since been released from the hospital.

The attack happened in a very remote area northeast of Snoqualmie and North Bend. The King County Sheriff's Office said the two people were riding bikes on a gravel road when the cougar attacked.

"They were riding when one hears a scream from the partner, look back, and sees they're being chased by a cougar," said Alan Myers with WDFW

Authorities said the cyclists did everything right. They got off their bikes, made noise and tried to scare the animal off. One even smacked it with his bike after it charged. The cougar ran off, but it returned and attacked when the cyclists got back on their bikes.

"He was able to get back on his bike after being mauled, and rode out of the area," said Sgt. Ryan Abbott with the King County Sheriff's Office. "Before leaving the area, he looked back and saw his friend, and the cougar was attacking his friend. He wasn't able to go back and help [Brooks]."

Brooks ran into the woods to try escaping the attack, but the animal chased after them. Officers found their bike on the road, then later found the cougar in the woods with Brooks. A deputy fired a shot at the cougar and it ran off. WDFW officers eventually found the animal again, shooting and killing it.

This was only the second time in the past 100 years where a cougar killed a human in Washington state, according to Fish and Wildlife.

Gary Koehler, a former Fish and Wildlife research scientist for DFW, said cougars are typically wary of humans.

"This is an extremely rare event," said Koehler. "So nobody knows what may have triggered this event, but it will probably remain a mystery."

WATCH: Wildlife expert says fatal cougar attack is 'extremely rare'