Associated Press
In this 2008 file photo, Purdue University virologist Suresh Mittal looks at petri dishes in his lab. The U.S. government has paid scientists to figure out how the deadly bird flu virus might mutate to become a bigger threat to people.

WASHINGTON — The U.S. government paid scientists to figure out how the deadly bird flu virus might mutate to become a bigger threat to people — and two labs succeeded in creating new strains that are easier to spread.

On Tuesday, federal officials took the unprecedented step of asking those scientists not to publicize all the details of how they did it.

The worry: That this research with lots of potential to help the public might also be hijacked by would-be bioterrorists. The labs found that it appears easier than scientists had thought for the so-called H5N1 bird flu to evolve in a way that lets it spread easily between at least some mammals.

"It wasn't an easy decision," said Dr. Anthony Fauci, infectious diseases chief at the National Institutes of Health, which funded the original research.

The scary-sounding viruses are locked in high-security labs as researchers at the Erasmus University Medical Center in the Netherlands and the University of Wisconsin-Madison prepare to publish their findings in leading scientific journals. That's the way scientists share their work so that their colleagues can build on it, perhaps creating better ways to monitor bird flu in the wild, for example.

But biosecurity advisers to the government recommended that the journals Science and Nature publish only the general discoveries, not the full blueprint for these man-made strains. Tuesday, the government announced that it agreed and made the request.

In statements, the two research teams say they're making some changes, if reluctantly.