Beaver, Pa. ? A hepatitis A outbreak that has killed three people and sickened nearly 600 others who ate at a Chi-Chi’s Mexican restaurant was probably caused by green onions from Mexico, health officials said Friday. But how the scallions became tainted remains unclear.

Of the 575 or more hepatitis A cases, most involved people who ate mild salsa or two particular entrees at the restaurant 25 miles from Pittsburgh, state officials said. A report issued later by the Centers for Disease Control and Prevention also cited the mild salsa, but no entrees.

“All the evidence suggests that people had direct contact with the green onions,” said Joel Hersh, epidemiology director for the state Health Department.

It is the nation’s biggest known outbreak of hepatitis A.

The contamination could have been caused by anything from a sewage leak in a farm field to feces in a shipper’s truck, health officials said.

The restaurant at the Beaver Valley Mall has remained closed since the outbreak was confirmed Nov. 3.

It was not immediately known whether the green onions, or scallions, came from the same green onion suppliers suspected in smaller hepatitis A outbreaks in September in Tennessee and Georgia. Health officials have said the hepatitis strains in all three states are very similar.

Health officials in Pennsylvania initially suspected that Chi-Chi’s employees had failed to wash their hands after using the bathroom — another common way that hepatitis A is spread. But suspicion soon fell on the green onions, and last weekend the Food and Drug Administration issued a national advisory saying purchased green onions should not be eaten raw.

By then, Chi-Chi’s had pulled green onions from its 99 other restaurants in 17 states from Minnesota to the mid-Atlantic.

Unlike Chi-Chi’s hot salsa, which is packaged before it arrives at the restaurant, the mild salsa is made partly on site, health officials said.