IOWA CITY, Iowa (AP) — Responding to a major case of research misconduct, federal prosecutors have taken the rare step of filing charges against a scientist after he admitted falsifying data that led to millions in grants and hopes of a breakthrough in AIDS vaccine research.

Investigators say former Iowa State University laboratory manager Dong-Pyou Han has confessed to spiking samples of rabbit blood with human antibodies to make an experimental HIV vaccine appear to have great promise. After years of work and millions in National Institutes of Health grants, another laboratory uncovered irregularities that suggested the results — once hailed as groundbreaking — were bogus.

Han was indicted last week on four counts of making false statements, each of which carries up to five years in prison. He was set to be arraigned Tuesday in Des Moines, but he didn't show up due to an apparent paperwork mix-up. A prosecutor said Han will be given another chance to appear next week.

Han, 57, didn't return a message left at his home in Cleveland, where he's been living since resigning from the university last fall. A native of South Korea, he surrendered his passport following his arrest and initial court appearance in Ohio last week.

Experts said the fraud was extraordinary and that charges are rarely brought in such cases. The National Institutes of Health said it's reviewing what impact the case has had on the research it funds.

"It's an important case because it is extremely rare for scientists found to have committed fraud to be held accountable by the actual criminal justice system," said Ivan Oransky, co-founder of Retraction Watch, which tracks research misconduct.

Oransky, a journalist who also has a medical degree, said there have been only a handful of similar prosecutions in the last 30 years.

He said Han's case was "particularly brazen" and noted that charges are rarely brought because the U.S. Office of Research Integrity, which investigates misconduct, doesn't have prosecution authority, and most cases involve smaller amounts of money.

"It's a pretty extraordinary case involving clear, intentional falsification," added Mike Carome, a consumer advocate and director of Public Citizen's Health Research Group. "The wool was pulled over many people's eyes."

Carome noted that Han's misconduct wasted tax dollars and caused researchers to chase a false lead. He said such cases also undermine the public's trust in researchers.

Finding an HIV vaccine remains a top international scientific priority. A 2009 study in Thailand is the only one ever to show a modest success, protecting about a third of recipients against infection. That's not good enough for general use, so researchers continue exploring numerous approaches.

According to the indictment, Han's misconduct caused colleagues to make false statements in a federal grant application and progress reports to NIH.

The NIH paid out $5 million under that grant as of earlier this month. Iowa State has agreed to pay back NIH nearly $500,000 for the cost of Han's salary.

Han's misconduct dates to when he worked at Case Western Reserve University in Cleveland under Michael Cho, who was leading a team testing an experimental HIV vaccine on rabbits.

Starting in 2008, Cho's team received initial NIH funding for the work. Cho reported soon that his vaccine was causing rabbits to develop antibodies to HIV, which left NIH officials "flabbergasted," according to a criminal complaint against Han.

Cho's team sent blood samples in 2009 to Duke University researchers, who verified the apparent positive impact on the vaccinated rabbits. The confirmation was seen as "a major breakthrough in HIV/AIDS vaccine research," according to the complaint.

Iowa State recruited Cho in 2009, and with his team — including Han — he soon received a five-year NIH grant to continue the research. The team kept reporting progress. But in January 2013, a team at Harvard University found the promising results had been achieved with rabbit blood spiked with human antibodies.

An investigation by Iowa State pinpointed Han, after he was caught sending more spiked samples to Duke University. In a Sept. 30, 2013 confession letter, Han said he started the fraud in 2009 "because he wanted (results) to look better" and that he acted alone.

"I was foolish, coward, and not frank," he wrote.

Cho has said he was devastated and angered that he wasted years on the research, but he has vowed to continue his work. He has not been accused of any wrongdoing.

Stephen Brown, medical director for the AIDS Research Alliance, said the case highlights the fierce competition to win increasingly scarce NIH research funding.

"Han's case also indicates the need for greater transparency and oversight of the peer review funding process, which is cloaked in secrecy and often leads to large sums being given to favored organizations, despite a lack of output," Brown said in a statement.