He titled his missive, “Apology regarding the paper retractions.” He wrote: “I am deeply ashamed of the fact that two papers of which I am an author were found to contain multiple errors and, as a result, had to be retracted.”

Part of what so shamed him, he said, was his failure as a mentor. “As a deputy director of our center, with responsibility for nurturing young researchers, I feel a deep responsibility for what has happened, and plan to comply with whatever decision Riken [research institute] finally reaches regarding my own status.”

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He wouldn’t survive long enough to see what that decision would be. On Tuesday, Japanese police said a security guard at a building nearby Riken in Kobe, Japan, found him hanging from a rope attached to a staircase railing, according to the Associated Press. Notes were found near Sasai, who died two hours later at the hospital after suffering cardiac arrest. And one of them, according to Agence France-Presse, was addressed to his star pupil, Haruko Obokata, who investigators allege fraudulently produced the now-retracted papers.

“I can’t really say whether the articles contributed to” his death, Riken spokesman Jens Wilkinson told The Washington Post. “It’s certainly possible, but I don’t know what else was going on his life, so it’s very hard to speculate. … He had the stress from the retractions and there was also the prospect that he would face disciplinary action due to the finding of research misconduct.”

The alleged research misconduct occurred in the formulation of STAP — “stimulus-triggered acquisition of pluripotency” — a purported finding that originally blew everyone away. It claimed to show that stem cells could be made with a startlingly simple procedure of dripping blood cells into acid, and it was initially hailed as a major breakthrough that could lead to treatments for illnesses such as Parkinson’s disease.

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But problems with the research appeared almost immediately. The images in the studies didn’t seem quite genuine, and the language in it bore too great a resemblance to another study published a decade before. Accusations of plagiarism and cooked research soon followed — as did the calls for a retraction, which followed last month.

Sasai, who was Obokata’s mentor, was celebrated in a 2012 Nature profile as the “brainmaker.” The piece portrayed the scientist as a serious man with a reserved demeanor and a gift for ingenuity. He had also just published an article announcing the creation of a human-eye precursor with stem cells. In the work, he had cultivated a novel way of research, in which he gave cells free rein to “do their thing,” according to the Nature profile. He was then enjoying accolade after accolade and was planning on growing brain parts. “These papers make for the most additive series of stem-cell papers in recent years,” Luc Leyns, a stem-cell scientist in Brussels, told Nature.

But less than two years later, he stood before a throng of microphones, trying to explain what had gone so wrong with Obokata’s STAP cell research. He said the papers should be retracted but maintained that the founding “hypothesis” remained viable. “STAP cells will have to be replicated,” the Japan Times quotes him saying. “I believe it is worth doing so with respect to the potential of this research.”

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He claimed he only advised Obokata’s research during the final stages of the work — and hadn’t known about the data errors. But an investigative Riken panel, which cleared him of misconduct, nonetheless said he bore a “heavy responsibility” for not catching the mistakes as the senior researcher. It also accused Obokata of “research misconduct,” according to the Japan Times.

Sasai was devastated by the retraction, saying in his letter that “it has become increasingly difficult to call the STAP phenomenon even a promising hypothesis.”

But Obokata hasn’t given up. She’s trying to replicate the research under video surveillance at the same institution where Sasai apparently committed suicide.