US journal Science has begun investigating a 2004 paper it published that first brought embattled cloning pioneer Hwang Woo-suk to the spotlight.

The journal said in a statement Tuesday that the editors of Science are reviewing both the 2004 and 2005 papers from Hwang's laboratory in light of new questions about the authenticity of images in the 2004 paper.

The journal’s probe is expected to focus on two vital photographs that Hwang used in his 2004 paper. Local scientists have raised allegations that the photos used in the 2004 study appear identical to photos published previously in another journal on an unrelated topic.

Hwang maintains his central findings, published in two Science papers in 2004 and 2005, are legitimate, though he has admitted to ethical lapses and sloppy reporting and has asked for the 2005 cloning paper to be retracted.

A Science spokeswoman said the journal was awaiting results from investigations at Seoul National University and the University of Pittsburgh, which employs Gerald Schatten who served as senior author on Hwang's 2005 cloning paper. Science has yet to formally retract that paper.