The skiing world is embroiled in a "blood doping" scandal after leaked footage showed a skier giving himself an alleged blood transfusion.
Austrian cross-country skier Max Hauke was as one of five athletes arrested in Seefeld, Austria, which is hosting the Nordic World Ski Championships.
Blood doping involves re-injecting an athlete's own blood to boost red-blood cell concentration.
The arrests have sent shockwaves through the skiing world.
A police officer is also reportedly facing investigation for passing the video of Mr Hauke to the press.
Here's what you need to know.
What is 'blood doping'?
Blood doping refers to various techniques used to boost the concentration of oxygen-carrying red blood cells, which can improve athletic performance.
Mr Hauke was allegedly using a transfusion - where an athlete's own blood is taken out before an event and re-injected during competition, boosting the red-blood cell count.
That allows the blood to transport more oxygen to the muscles, increasing an athlete's stamina.
The three most common substances and methods of blood doping are synthetic oxygen carriers, blood transfusions, and erythropoietin (EPO). All three are banned by the World Anti-Doping Agency (Wada).
EPO is a hormone that is produced naturally in the body and stimulates red blood cell production. Abusing it carries health risks because it thickens the blood, leading to an increased risk of fatal blood clots, heart disease, and strokes.
What does the video of Max Hauke show?
Some readers may find the clip difficult to watch. In the video, Mr Hauke is seen sitting on a sofa moments after police entered the room. He appears to be mid-transfusion.
The video was reportedly filmed by a police officer, who later leaked it to the Norwegian broadcaster NRK. Mr Hauke has yet to respond publicly to the allegations.
Who's been arrested?
In total nine people were arrested in a series of raids on 16 properties, carried out by 120 officers from both German and Austrian police, in what has now been dubbed "Operation Bloodletting".
Nine of the raided properties were in Erfurt, Germany, where officers allegedly found a blood doping laboratory. One Kazakh, two Estonian and two Austrian athletes were detained.
Among them were Mr Hauke, an Olympic skier who represented his country in Sochi in 2014, and fellow Austrian skier Dominik Baldauf.
The two Estonian athletes - Karel Tammjärv and Andreas Veerpalu - were both released on Thursday evening, while Mr Baldauf, Mr Hauke, and Kazakh skier Alexey Poltoranin were freed earlier in the day.
A 40-year-old sports doctor, named only as "Mark S", has also been arrested. He is believed to be a central figure in the doping ring.
Plus, Austrian broadcaster ORF reported that the police officer who had leaked the video of Mr Hauke was now being investigated as well, and could face disciplinary and criminal proceedings for giving the clip to the press. The unnamed officer was also apparently let go from his post "with immediate effect".
Has anyone confessed?
In a press conference on Friday, Mr Tammjärv admitted he had started seeing his doping doctor back in 2016.
"I made that decision myself that I wanted to get help in the form of blood doping," Mr Tammjärv said, adding that he had first withdrawn blood that summer, and had completed his first transfusion during the Nordic World Ski Championships in Lahti, Finland, the following year.
Innsbruck regional prosecutor Hansjörg Mayr said in a statement that Mr Baldauf, Mr Hauke and Mr Poltoranin had also "admitted to using blood doping and gave comprehensive and in-depth information to investigators".
What about resignations?
Trond Nystad, the Austrian team coach, has quit his job - with effect from the end of the championship on Sunday.
He told Norwegian outlet VG he had made the decision after watching the leaked footage of Mr Hauke's blood doping, which reportedly made him feel so sick that he physically threw up.
He said he now had "no desire to work with the Austrian ski club anymore". But he said he had had no idea about the doping before this week: "If I had [had suspicions], I'd have reported it. I have zero tolerance for doping."
How has the rest of the skiing world reacted?
Like Mr Nystad, Mr Hauke's British training partner Andrew Young said he had felt physically sick watching the video. "I'm getting nauseous. It's disgusting to look at," he told NRK.
Other skiers were similarly shocked by the allegations. Andrew Musgrave, a Scottish skier who had hit a personal best in Seefeld on Wednesday, said the news had overshadowed one of his "best ever classic races".
And GB Snowsport's chief executive Vicky Gosling said it was "extremely disappointing", and a sign that "doping remains a serious issue in elite sport".
"The athletes and staff on the cross-country skiing World Cup circuit all know each other very well so today's news has been both shocking and disappointing," she said.