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. In October 2014, all federal funding was halted on efforts to make three viruses more dangerous: the flu virus, and those causing Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS). But the new regulations apply to any pathogen that could potentially cause a pandemic. There has been a long, fierce debate about projects known as gain of function research intended to make pathogens more deadly or more transmissible. Tensions rose in 2014 after the Centers for Disease Control and Prevention accidentally exposed lab workers to anthrax and shipped a deadly flu virus to a laboratory that had asked for a benign strain. That year, the N.I.H. also found vials of smallpox in a freezer that had been forgotten for 50 years. When the moratorium was imposed, it effectively halted 21 projects. In the three years since, the N.I.H. created exceptions that funded ten of those projects.