The cyber-espionage group Fancy Bears, which made headlines worldwide when it published the private medical certificates of athletes including Bradley Wiggins, Serena Williams and Simone Biles, has struck again by hacking into the confidential emails of senior anti-doping officials.

The emails – which have been taken in separate attacks on the World Anti-Doping Agency (Wada) and the United States Anti-Doping Agency (Usada) – include conversations about high-profile US athletes who might have unusual blood values, a tip-off that at least two US Olympians were taking cocaine to lose weight before the Rio Games and strong suggestions that a non-American athlete, who has not been banned, had a blood transfusion before a major race.

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The emails, which also contain the personal details of the Wada president, Sir Craig Reedie – including his home phone number – will raise further concerns about the ability of anti-doping agencies to protect confidential data. In September Fancy Bears used a phishing attack to access Wada’s athlete management system before publishing details of athletes’ therapeutic use exemptions (TUEs) – certificates that allow athletes to use medication on Wada’s prohibited substances list because of an illness or condition.

In a separate development Fancy Bears has also released more names of British athletes who have taken therapeutic use exemptions including the sprinter Adam Gemili, who British Athletics has confirmed was given an intravenous painkiller after tearing his hamstring in the 100m at the Birmingham Diamond League last year, and the Team Sky rider Ian Stannard, who was allowed to take furosemide before a kidney scan after damaging a vertebra in 2014. There is no suggestion of wrongdoing on the part of either athlete.

It is the revelations in the Usada and Wada emails that are likely to provoke the greatest interest. They include:

• An email about an anonymous tip-off about a group of American sportspeople, some of whom later competed in Rio, using cocaine “specifically for weight loss purposes”. The source, who suggests that the practice has gone on with the knowledge of a coach, is described as “likely credible”. The internal email says that the athletes will be urgently target-tested – but also points out that cocaine will not necessarily be an anti-doping rule violation as the drug is not banned out of competition.

• A Usada member flagging up a number of high-profile US athletes as having “possible incorrect [blood parameter] values”. The email includes a number of athletes whose haematocrit, human growth hormone and mean corpuscular volume levels are unusual, although the author adds: “Granted this is only blood parameters but let me know if you think these are errors or if the values really are that high/low.”

• The vitamins and medication that well-known athletes told drug-testers they were using before an out-of-competition test. For instance, it was disclosed one athlete was on Advair, Synthroid, vitamin D, Allegra, Cytomel, Singulair and Tylenol, some of which are powerful and legal asthma and thyroid medication.

• A distance athlete who competed in Rio was flagged up as having suspicious blood values in the weeks before the Olympics. As one email puts it: “She was heavily tested over the past weeks, and from her haematological passport its [sic] clear [she] was blood doping from tests earlier this month. The passport is now under review and will certainly be reviewed as likely doping.”

In a statement Wada said: “The criminal leak of this information – which is regular correspondence on anti-doping operational matters from Wada staff – illustrates nothing new and is once again part of a continued effort to discredit Wada and the broader anti-doping system. Furthermore, this activity is part of an attempt to distract from the real issue: the serious breaches to world anti-doping rules that were evidenced in the McLaren report.”

The Guardian understands the hack of Usada data is suspected to have come about after an official from the organisation used a public computer while at the Paralympics in September. Usada, which has been working with the FBI’s security task force, has also now hired a private security firm to monitor its cyber efforts.

In a statement, Usada spokesman Ryan Madden said: “While in a lot of ways these emails only further demonstrate the high standard to which athletes in the US are held, what is concerning to us – as well as anyone else who truly values the spirit of Olympism – is the clear and malicious violation of athletes’ rights.

“What we hope doesn’t get lost in the headlines is that these athletes have done everything asked of them, have been held to the highest standard and are now being forced to watch as a cyber-espionage group attempts to create fake news.”

Previously Fancy Bears have published documents showing that over 100 athletes have taken TUEs. Among them were revelations that Serena Williams had taken the restricted drugs prednisone, prednisolone, methylprednisone, hydromorphone and oxycodone between 2010 and 2015, while Biles was given methylphenidate for attention-deficit disorder and Wiggins was allowed to take the powerful corticosteroid triamcinolone before the 2011 and 2012 Tour de France and the 2013 Giro D’Italia. In all cases, the athletes had been granted TUEs by the relevant international sports federations and national anti-doping organisations and Wada insisted that no wrongdoing had occurred.