World champion sprinter Justin Gatlin says he is "shocked and surprised" at doping allegations made against his coach and an agent.
Athletics anti-doping officials have begun an investigation into what the president of the sport's governing body called "serious allegations" about Dennis Mitchell and Robert Wagner.
The Daily Telegraph said Wagner - an agent linked to Gatlin - offered to "illicitly supply performance-enhancing drugs" to undercover reporters.
A video released by the Telegraph features a man the newspaper says is Wagner insinuating Gatlin is taking banned drugs, "just like every other sprinter in America".
The paper also said Gatlin's coach, former Olympic gold medallist Mitchell, told reporters that athletes are able to get away with doping because the drugs they use cannot be detected by tests.
Both deny the allegations.
Writing on Instagram on Tuesday, Gatlin said he "fired" Mitchell "as soon as I found out about this".
The 35-year-old American said he is "not using and have not used" performance-enhancing drugs.
He added: "All legal options are on the table as I will not allow others to lie about me like this."
The Telegraph said its journalists had posed as representatives of a film company wanting to make a sports film who were looking for a coach to train their star to look like an athlete.
The Athletics Integrity Unit (AIU) - set up by the sport's world governing body the IAAF - and the US Anti-Doping Agency (Usada) said they had opened an investigation into the claims.
"Investigations stemming from tips and whistleblowers play a critical role in anti-doping efforts," Usada said in a statement. "We are presently co-ordinating with the Athletics Integrity Unit in order to investigate these claims fully.
"As with all investigations, we encourage individuals with information to come forward as an important tool to help protect clean athletes. Importantly, individuals are innocent unless and until the established process determines otherwise. It's only fair to let due process occur before jumping to any conclusions."
IAAF president Lord Coe said: "These allegations are extremely serious and I know the independent Athletics Integrity Unit will investigate in accordance with its mandate."
Doping allegations 'disappointing' - Wada president Reedie
The World Anti-Doping Agency (Wada) said it would monitor the AIU investigation and offer support.
"I hope that first of all that the journalists from the Telegraph will make their information available to the investigators. We will monitor and offer assistance where we can," said Wada president Sir Craig Reedie.
"I think the investigation will concentrate on the allegation that everybody is using performance enhancing drugs. It's an easy allegation to make but it needs to be tied down - I'm not sure the evidence exists to support it."
Gatlin, who has served two doping bans, won 100m gold at August's World Championships in London, beating Usain Bolt in the Jamaican's final individual 100m race before retiring.
Gatlin's legal representatives said the sprinter had more than five years' worth of official drugs tests to show "he has never tested positive for any banned substance", the paper reported.
In a statement to the Telegraph, Mitchell said: "I never suggested in any way that any of my current athletes used any banned substances or that I was familiar with training any of my current athletes with those substances."
Wagner told the paper: "I wasn't involved in doping. Obviously I played along because I knew what was going on. I had to get them hooked."