Brevik and equipment manager Adrian Livelten were suspended by the Norwegian Ski Federation this week.
Brevik told local media he was “terribly sorry for what we did” while Livelten apologised for the situation Lindvik and Forfang “have found themselves in through no fault of their own”.
“What we did with the suits should never have happened and is an action I will regret for the rest of my life,” Livelten said.
“We have always done what we can to optimise the suits within the regulations, but cheating is completely unacceptable.”
The FIS has seized all Norwegian jumpsuits from the World Ski Championships for reinspection.
FIS secretary general Michel Vion said the situation was “disturbing and disappointing”, and the investigation would “leave no stone unturned” to ensure “respect and fairness prevail”.
“Ski jumping is a discipline grounded in precision, in which equipment plays an important role,” Vion added.
“This is why, year after year, we have a strong focus on reviewing equipment regulations and controls, to ensure that competitors are on a level playing field.”
Lindvik won gold in the men’s large hill individual at the 2022 Winter Olympics in Beijing, while Forfang claimed a silver medal at the 2018 Games in Pyeongchang in the normal hill individual.