AP

MOSCOW (AP) — There "simply may not be sufficient evidence" to secure bans for many of the Russian athletes implicated in an alleged state-backed doping scheme, the World Anti-Doping Agency said on Saturday.

WADA investigator Richard McLaren reported in December that more than 1,000 Russian athletes may have benefited from a cover-up which allegedly included tampering with samples given by 12 medalists at the Sochi Olympics.

It's proving hard to turn McLaren's report, much of it based on records leaked by a former Russian drug-test laboratory head, into watertight cases against individuals.

WADA said in a statement that the destruction of more than 1,000 samples in a Moscow laboratory and a lack of Russian co-operation mean "there simply may not be sufficient evidence required to sanction ... some of the individual athletes identified in the report."

WADA didn't say how many cases are affected. Some winter sports have allowed Russians who were implicated in McLaren's report to keep competing pending investigations. In January, the International Biathlon Union cleared 22 of 29 Russians who had been implicated in its sport.

McLaren's investigation wasn't originally intended to build cases against individuals, but to focus on whether there was a wider conspiracy in Russian sports.

There's little clear precedent for banning athletes whose samples were tampered with, especially if they argue samples could have been swapped without their knowledge.

However, WADA says McLaren's report has already helped to catch Russian drug cheats in a different way, as intelligence for Olympic officials running their own program of retesting samples from past Olympics, launched last year.

"Information from the McLaren report helped the IOC target its reanalysis program of samples from the 2008, 2010 and 2012 Olympic Games, which has so far resulted in sanctions against 38 Russian athletes," WADA said.

More than 100 athletes from various countries have been disqualified from the 2008 and 2012 Olympics.