LONDON (AP) – Justin Gatlin’s association with coach Trevor Graham is the biggest obstacle to proving that he’s not a drug cheat, former Olympic champion sprinter Michael Johnson said Monday.

Writing a column in The Daily Telegraph, Johnson said his initial reaction to hearing that Gatlin had tested positive for banned substances was “not one of shock or surprise.”

“Not because I have suspected that Justin was doping – I’ve always had a lot of respect for him – but there has always been one thing about him that has bothered me: his association with coach Trevor Graham,” Johnson said.

Graham, who did not return messages seeking comment, is a key figure in the BALCO steroid investigation and has coached several athletes who have been suspended in doping cases, including former 100-meter world-record holder Tim Montgomery.

“Even if Gatlin is innocent, now he will be suspected forever and he is about to see the danger of his continued association with Graham, because that association almost guarantees that no one will give him the benefit of the doubt,” Johnson wrote.

Johnson won five Olympic gold medals and still holds the world record for the 400 meters and 200 meters. He’s the agent for reigning Olympic and world 400-meter champion Jeremy Wariner.

Gatlin’s mother, Janet, said Monday evening that her son was staying at the family’s Pensacola, Fla., home and “was doing as well as can be expected.”

Asked if she was confident her son would be cleared of doping allegations, Janet Gatlin referred questions to the sprinter’s attorney and publicist. She said her son had no statement beyond the one he issued on Saturday.

Graham said Sunday that Gatlin, the reigning Olympic and world champion and co-world record holder in the 100, was victim of a massage therapist who rubbed testosterone cream on Gatlin without his knowledge.

“That’s a new twist on an old excuse and Graham has no credibility,” Johnson said.

On Saturday, Gatlin acknowledged he had been informed by the U.S. Anti-Doping Agency that he tested positive for testosterone or other steroids after a relay race in Kansas in April.

Johnson said Gatlin should be banned for life unless he can prove his innocence.

He said Graham also should be banned for life for his involvement with athletes who tested positive while training under him.