BERLIN - Thirty athletes are being investigated in connection with a suspected doping affair involving a doctor in the city of Erfurt, the German anti-doping agency (NADA) said on Monday.

NADA said they were looking into each of the 30 cases where the doctor, once part of the Olympic base in Erfurt, is suspected of having used a blood infusion system after exposing the blood to UV rays.

The procedure is said to boost the oxygen content in the blood.

The doctor, named by the Erfurt prosecutor's office as Andreas Franke, has already been suspended. He has denied any wrongdoing, saying he used the procedure only to battle infections.

"Obviously we are looking into each case to see whether banned methods were used, no matter if it concerns Olympic champions or young athletes," NADA board director Andrea Gotzmann said in a statement. The list includes cyclists and speedskaters.

Gotzmann said NADA had received access to the prosecutor's documents on Monday which would help them to decide "which athletes we will go after".

German state television had said on Sunday that 28 athletes were on a list of patients the doctor had treated with the suspect procedure.

"The doctor is being accused of having taken blood out of athletes, treated it with UV rays and then re-injected it back into their bodies," said prosecutor's office spokesman Hannes Gruenseisen.

"So there is the initial suspicion that he used banned medical equipment for doping."

Under the World Anti-Doping Agency code, blood transfusions of any kind and size are banned unless there are specific medical reasons for them, approved under a special permit.