An FBI investigation has halted the operations of nuclear couriers based in Oak Ridge, where workers handling radioactive materials have raised health and safety concerns. A watchdog group says the government is trying to stifle whistleblowers.

The 62 nuclear bomb part couriers, who drive specially designed trucks and are trained in anti-terrorist tactics, have been idle for three weeks and continue to be paid, said Al Stotts, a Department of Energy spokesman based in Albuquerque, N.M.The FBI is investigating a possible breach of classified information among the couriers, Stotts said.

One issue is whether information was leaked to an ABC news crew that reportedly showed up at places where working couriers had stopped, The Knoxville News-Sentinel reported Saturday, citing unidentified sources familiar with the case.

Times and routes of nuclear weapons shipments are supposed to be kept secret.