IOWA CITY, Iowa — 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.