Federal officials on Tuesday ended a moratorium imposed three years ago on funding research that alters germs to make them more lethal.

Such work can now proceed, said Dr. Francis S. Collins, the head of the National Institutes of Health, but only if a scientific panel decides that the benefits justify the risks.

Some scientists are eager to pursue these studies because they may show, for example, how a bird flu could mutate to more easily infect humans, or could yield clues to making a better vaccine.

Critics say these researchers risk creating a monster germ that could escape the lab and seed a pandemic.