Japan's top research institute has hammered the final nail in the coffin of what was once billed as a ground-breaking stem cell study, dismissing it as flawed and saying the work could have been fabricated. The revelations come a week after a young researcher at the centre of the scandal, which has rocked the country's scientific establishment, said she would resign after failing to reproduce the successful conversion of an adult cell into a stem cell-like state, known as "STAP" cells. The failure marked a stunning fall from grace for Haruko Obokata, 31, whose co-researcher committed suicide amid the embarrassing scandal that prompted respected science journal Nature to retract an article detailing the research. On Friday the government-backed Riken institute, which sponsored the study, said embryonic stem cells had been added in the process of the research, hammering Obokata's contention that she had found an easier way to generate new stem cells in the lab. "But we can't conclude whether the mixing was done on purpose or by mistake nor can we conclude who did it," probe team chief Isao Katsura, head of the National Institute of Genetics, told a news briefing in Tokyo. In January, Riken trumpeted Obokata's simple method to re-programme adult cells to work like stem cells. The study was top news in Japan, where the photogenic Obokata, a Harvard-trained scientist, became a phenomenon. But media attention soon grew into scepticism as doubts emerged about Obokata's papers on Stimulus-Triggered Acquisition of Pluripotency (STAP). Mistakes were discovered in some data published in two papers, photograph captions were found to be misleading, and the work itself could not be repeated by other scientists. Embryonic stem cells are prototype "mother" cells found in early-stage embryos, with the potential to become any kind of tissue in the body.