The president of the government-backed Riken institute admitted Friday there were “grave errors” in two papers produced by its researchers on a possible method to create pluripotent stem cells but wouldn’t say whether the alleged irregularities were intentional.

Separately Friday, Haruko Obokata, lead author of the two papers on potentially groundbreaking stem cell research, and two of her co-authors, released a statement in which they said they are looking into the possibility of withdrawing the papers.

The papers, written by 14 Japanese and U.S. researchers about a procedure to reprogram stem cells, were published in the British science journal Nature in late January. If the papers are withdrawn, the findings themselves will also be retracted.

In announcing an interim report by its investigation committee on the allegations Friday in Tokyo, Riken President Ryoji Noyori said: “I’m sorry to have bothered and worried the public due to doubts on the papers on STAP (stimulus-triggered acquisition of pluripotency) published in Nature by researchers at Riken.”

Noyori, a Nobel Prize laureate in chemistry, said he instructed the institute to cooperate with outside stem cell researchers and provide them with the necessary information on the method to create the cells to verify its reproducibility and authenticity.

The investigative committee, headed by Riken scientist Shunsuke Ishii, found “inappropriate handling” of research data in two of the six items subject to the probe, it said.

Regarding the “unnatural appearance of colored cell parts” shown in one of the papers, however, the report said the “process of preparing these images did not constitute fabrication within the context of research misconduct.”

It also said there was no malice found in the resemblance of some of the images of mice placenta used in the paper, as it saw “nothing to contradict the explanation that one of the figures had inadvertently been left undeleted during the process of manuscript creation.”

The remaining four items need further investigation, the report said.

Obokata is one of the individuals being investigated by the committee, along with co-authors Yoshiki Sasai, Hitoshi Niwa and Teruhiko Wakayama.

On Friday, Obokata issued a joint statement of apology with Sasai and Niwa, saying, “We are contacting other co-authors and looking into the possibility of retracting the papers.”

To have an article withdrawn from the British journal, consent must be obtained in principle from all of the authors involved.

Charles Vacanti of Harvard Medical School, one of the authors, has not given his consent, saying he firmly believes the questions raised about the papers “do not affect our findings or conclusions.”

Masatoshi Takeichi, director of the Kobe-based Center for Developmental Biology at Riken, said Obokata, who belongs to the center, has suspended research due to the current situation.

The papers have come under scrutiny for various problems, including images resembling those used in Obokata’s doctoral dissertation in 2011. The images are said to be crucial to support the assertion that STAP cells were indeed produced.

Information from Kyodo added