By Chine Labbé

PARIS (Reuters) - French prosecutors investigating whether bribery was involved in the award of the 2021 world athletics championships to the U.S. city of Eugene are also looking into several other host city decisions, a source close to the case said on Friday.

The inquiry, one of a group of French probes into cash-for-votes allegations surrounding the award of sporting events around the world, is looking at decisions by the International Association of Athletics Federations (IAAF) to give their blue riband event to Doha (2019), London (2017), Beijing (2015) and Moscow (2013), the source told Reuters.

Other investigations already under way include two into the Olympic Games held at Rio de Janeiro in 2016 and due in Tokyo in 2020. "Six current or former members of the International Olympic Committee are suspected of receiving payments in exchange for their votes on sporting events," the source told Reuters.

The six include Namibian Frank Fredericks, who stepped down on Tuesday as head of a team evaluating bids for the 2024 Olympics, and former IAAF president Lamine Diack, who is already under investigation for corruption after allegations that he received bribes for suppressing positive doping tests of Russian athletes.

"The investigation goes back to Stuttgart," the source said, in reference to the World Athletics Final, the discontinued end-of-season finale which the German city hosted in 2006, 2007 and 2008.

The source said that Senegalese Diack and his son Papa Massata Diack, who is currently in Senegal, were "at the heart of the inquiry."

Fredericks said this week that a $300,000 payment he received from Papa Massata Diack on the day of the vote to choose the 2016 Olympics was for promoting athletics in Africa and the former sprinter has denied any wrongdoing.

French newspaper Le Monde said this week that Papa Massata Diack was paid $1.5 million dollars by a Brazilian businessman three days before the 2009 vote to award the 2016 Olympics to Rio.

Both Diacks have repeatedly denied taking any bribes.

(Reporting by Chine Labbe/Mitch Phillips; Editing by Toby Davis)