German anatomist Gunther von Hagens has said he cannot rule out the possibility that some bodies supplied for his Body Worlds show were executed prisoners.
Professor von Hagens says many of his corpses are from volunteers
He was responding in Frankfurt to an article in German magazine Der Spiegel which alleged he had used the bodies of Chinese people sentenced to death.
He said it was "highly improbable" that any such bodies had got through.
However, he said he would return seven Chinese bodies for burial because he could not prove they were not executed.
The Body Worlds show of specially preserved skinless bodies has caused controversy around the world.
"I have never used people who have been sentenced to death," he said.
"The Spiegel magazine asked me if I rule this out. No, of course I can't fully rule it out, because I don't know if the body of a man who was sentenced to death was ever unknowingly delivered to us.
"I think that it is highly improbable. I can practically rule it out."
He said that following the article he had asked that all the bodies from China be searched for head injuries.
"But I'm telling you, not even a bullet hole in the body can be proof that that person was sentenced to death. Out of all 650 bodies, seven had head injuries. This is completely normal, seeing the number of bodies."
He said he took the allegations seriously and would speak to the Chinese authorities.
The exhibitions, which have included the body of a dead child and a horse, have caused a stir in many countries attracting more than eight million visitors worldwide since they began in 1995.
In Frankfurt, state authorities have warned parents not to allow children younger than 14 to view the exhibit, which they said could "shock and frighten" them.
Last year Edinburgh City Council, in Scotland. turned down the exhibition on the grounds that some people could find it offensive.
Professor von Hagens says his cadavers are usually obtained through donations.
More than 4,500 people have also offered to give their bodies to Professor von Hagens for use in his displays after their deaths
In 2002, he brought the show to London. It was a huge success and attracted well over 550,000 visitors.