An investigative panel under the state-backed Riken research institute said Friday that the debunked STAP cells generated by one of its scientists were likely created instead from embryonic stem cells.

It also said it identified two further instances of research misconduct by the lead author of the reports, Haruko Obokata.

The panel “concluded STAP stem cells . . . were derived from ES cells, based on examinations of the remaining samples,” Isao Katsura, director general of the National Institute of Genetics and head of the seven-member panel, told reporters in Tokyo.

“The papers’ core claims are refuted, since the introduction of ES cells was indicated,” Katsura said.

Katsura said tissuelike growths known as teratomas and artificially created, or chimeric, mice claimed to have been grown from STAP cells were probably grown from ES cells instead. The formation of teratomas and chimeras is considered key to proving the existence of pluripotent stem cells, biological building blocks that can be coaxed into developing into any type of tissue.

However, the panel was unable to determine who introduced the embryonic stem cells, and whether it happened deliberately or by accident, citing insufficient evidence.

Moreover, the panel, which had been investigating the two papers on stimulus-triggered acquisition of pluripotency published in the British science journal Nature, found two more instances of research misconduct by Obokata in connection with data in one of the papers.

“We concluded Obokata fabricated data for two charts on the growth curves of (‘STAP’) cells and DNA methylation,” Katsura said.

A separate Riken panel earlier this year found two separate instances of misconduct. Those findings, and related allegations, prompted Riken to set up the panel headed by Katsura in September. Obokata resigned from the institute on Sunday.

Meanwhile, Katsura cleared Teruhiko Wakayama and Hitoshi Niwa, two co-authors, of misconduct. He added that the panel did not investigate Yoshiki Sasai, a co-author who had overseen Obokata’s writing and who committed suicide in August.

The papers published in Nature in January said the researchers had managed to reprogram adult mice cells as pluripotent cells by simply soaking them in mildly acidic liquid.

Discrepancies were soon alleged, and in July the journal retracted the reports.

After a news conference by the panel, Riken President Ryoji Noyori issued a statement apologizing for the loss of trust caused by the scandal.

He said Riken’s disciplinary committee will resume its work. It was set up in May in after the scandal broke but has remained dormant pending the outcome of the probe.

Mutsuhiro Arinobu, Riken’s executive director in charge of compliance, declined to comment on who could be penalized, saying it is up to the disciplinary committee to decide that.

Meanwhile, Maki Kawai, executive director in charge of research affairs at Riken, said it had been unable to reach Obokata, and thus had not conveyed its findings to her.