Controversial American sprinter Justin Gatlin has released letters detailing the extent of his co-operation with US anti-doping investigators and apologies made for his own offences.

Gatlin has served two doping bans during his career and endured persistently negative coverage during the World Athletics Championships in Beijing, where he claimed silver medals in the 100 and 200 metres behind Usain Bolt.

The 33-year-old entered the championships as the fastest man of 2015 and many pundits had admitted fears that athletics' reputation could be damaged by a Gatlin victory in the blue riband events - a prospect incoming IAAF president Sebastian Coe described as making him feel "queasy".

A perceived lack of contrition from Gatlin over his doping offences has contributed to his low public stock and he boycotted interviews with the BBC in Beijing, taking exception to the UK broadcaster's coverage of his battle with Bolt which he believed had been unhelpfully framed as one of "good versus evil".

But in letters seen by the Guardian newspaper, Gatlin outlines how he felt "sincerely remorseful" over his doping past and the steps taken by the sprinter to help ensure a clean sport are explained.

In one letter addressed to outgoing IAAF president Lamine Diack and his then senior vice-president Sergey Bubka, Gatlin wrote: "I am sincerely remorseful and it continues to be my mission to be a positive role model mentoring to athletes to avoid the dangers and public and personal humiliation of doping. And the harm it brings to the sport of athletics.

"I have co-operated fully with the United States federal investigation to clean up our sport of track and field working toward it becoming drug free." Gatlin added that he wanted to "rid our sport of drugs and all those who help supply them".

In a letter written to support Gatlin's bid to return to track and field following his four-year suspension spanning 2006 to 2010, the United States Track and Field Federation told the IAAF that he had gone into colleges to give talks on personal responsibility to students and warn them of the consequences of anti-doping rules.

USATF added that Gatlin maintained a regular dialogue with them throughout his suspension in order to stay informed on when his next opportunity to speak to young athletes at such events would come around.

Gatlin's agent Renaldo Nehemiah told the Guardian of his frustration that the IAAF has not taken a public stance to temper the current ill-feeling towards his client.

Nehemiah said: "When people say he never apologised, I say: 'You haven't done your homework.' And the IAAF, who know this, have never come out and said anything, which I am very sad about. 

"Justin has apologised. What is he supposed to do, go to every country and say sorry?"