A Japanese scientist who played an instrumental role in two discredited studies about a new type of stem cells hanged himself at his research institute in Kobe, according to media reports there.

Yoshiki Sasai, a deputy director of the RIKEN Center for Developmental Biology, left behind five apparent suicide notes, Japan Times reported Tuesday.

I am overcome with grief at this terrible news, RIKEN President Ryoji Noyori said in a statement released Tuesday. The scientific world has lost a talented and dedicated researcher, who earned our deep respect for the advanced research he carried out over many years. I would like to express my deepest condolences to Dr. Sasais family and colleagues.

Sasai was a coauthor on two papers published in Nature that purported to offer a quick and simple way of making highly versatile stem cells. Instead of destroying embryos or tinkering with their DNA, the scientists said they produced their flexible cells by stressing them out in an acid bath for 30 minutes and then spinning them in a centrifuge for 5 minutes.

At first, scientists hailed the creation of the so-called stimulus-triggered acquisition of pluripotency, or STAP, stem cells. But within days, serious questions arose about the researchers methods, leading to a RIKEN investigation that found several instances of scientific misconduct on that part of study leader Haruko Obokata, a rising scientist at RIKEN.

Both studies were retracted in July.

Sasai was Obokatas supervisor and was supposed to oversee her writing, Japan Times reported. RIKEN faulted Sasai for failing to check the data used in the study and for providing weak oversight that allowed Obokata to submit a manuscript with manipulated images and other serious problems.

"Research misconduct occurred due to a young researcher's lack of experience and awareness of the importance of research ethics, the lack of leadership among researchers to help her, and a lack of mutual verification among groups," Noyori said when RIKEN announced the results of its investigation in April.

Sasai appeared to take these criticisms to heart. He offered a very contrite statement when the studies were retracted.

As a researcher, I am deeply ashamed of the fact that two papers of which I was an author were found to contain multiple errors and, as a result, had to be retracted, he wrote. I also deeply regret the fact that as a coauthor, I was not able to identify these errors beforehand and to exercise my leadership to prevent this regrettable situation, including misconduct, from occurring. I apologize wholeheartedly for the confusion and disappointment that this situation has caused.

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STAP stem cell controversy ends in suicide for Japanese scientist