Two Austrian athletes have received kidnap threats if they should compete in the Sochi Winter Olympics, which heightens security concerns at the Games and triggered an investigation by counter-terrorism experts, The Guardian reports.

The threats were made anonymously in a letter to the Austrian national Olympic committee, and sources have identified the competitors as Alpine skier Bernadette Schild and skeleton racer Janine Flock.

Threatening letters were sent last month to delegations in several countries, but the International Olympic Committee deemed that they posed no danger.