Some portions of purportedly trailblazing stem cell research papers by Riken researchers and others were fraudulent, the government-backed institute acknowledged Tuesday.

A Riken panel investigating allegations leveled at the papers on so-called STAP cells found that two sets of images used in the papers were either manipulated or fabricated by Haruko Obokata, the Riken researcher who was the papers’ lead author.

The panel declined to say, however, at a press conference whether STAP cells — short for stimulus-triggered acquisition of pluripotency cells — really exist or not.

The panel also determined that three other co-authors of the papers, including Yoshiki Sasai, deputy head of the Riken Center for Developmental Biology, one of Riken’s research units, to which Obokata also belongs, and Teruhiko Wakayama, a professor at the University of Yamanashi, had not done anything fraudulent.

But the panel said their responsibility is “grave,” in that they failed to verify the contents of the papers.

In late January, two research papers were published in the journal Nature on STAP cells, a purportedly revolutionary method of producing cells that can turn into various kinds of cells.

The researchers led by Obokata claimed they can be produced by subjecting somatic cells from a mouse to stress through exposing them to acid liquid.

Allegations soon surfaced on the Internet that the papers used collaged images and misappropriated data from Obokata’s doctoral thesis.

In the interim report of its probe, Riken said the STAP cell papers contained “serious errors.” Of six allegations about the papers, the institute dismissed two but noted the remaining possibility of data manipulation and plagiarism.