World athletics' governing body feared Russian doping was so out of control that athletes could have died - six years before the country was banned from international competition.
Documents uncovered by the Associated Press show the International Association of Athletics Federations (IAAF) warned Russian athletics chiefs in 2009 that the blood levels of their athletes were "putting their health and even their own lives in very serious danger".
The 2009 letter from the IAAF to Valentin Balakhnichev, then the president of the All-Russia Athletic Federation (Araf), said results from tests at that year's World Championships in Berlin were "startling".
Balakhnichev was banned for life last week for breaching anti-doping violations.
On Thursday, a World Anti-Doping Authority (Wada) independent report is expected to reveal details of the IAAF's alleged complicity in covering up doping in the sport.
The first part of the report, published in November, led to Russia's athletes being banned from international competition because of "state-sponsored doping".
Russia won 13 medals at the 2009 event, but its athletes recorded "some of the highest values ever seen since the IAAF started testing", said the letter, written by the IAAF's general secretary Pierre Weiss.
The results "strongly suggest a systematic abuse of blood doping or EPO products", Weiss wrote.
The correspondence took place before the IAAF introduced the blood passport, which monitors abnormal results over a longer period of time to identify and ban cheats.
At that time, the results on their own were not enough to sanction athletes and it was down to the country's own anti-doping authorities to tackle cheats.
According to news agency AP, further correspondence before the 2012 London Olympics showed the IAAF proposed "discreet" handling of doping cases for less well-known Russian athletes.
The athletes could be removed from competition, with the world governing body not publishing the sanction, and they would be given a two-year ban, rather than four, if they agreed to the deal.
But the note said it would be "impossible" to take this approach with Russia's elite athletes without their absence from competition raising questions, so their bans would have to be made public.
The IAAF said these proposals were never put into action, and Balakhnichev told AP they never reached him.
The documents show that the IAAF already had grave concerns six years before it took action against Russia.
In the 2009 letter, Weiss asks Balakhnichev what internal sanctions the athletes with suspicious blood tests will face. He says the situation is "so serious" that "immediate and drastic action" is needed.
Yet it took the publication of the Wada report, based on the evidence of a 2014 German TV documentary, for the IAAF to rule that "the whole system has failed the athletes, not just in Russia, but around the world".
IAAF chairman Lord Coe said at the time: "This has been a shameful wake-up call."
