At least two people were killed and dozens sickened by E. coli outbreaks in Canada and the United States that the authorities in Canada have linked to romaine lettuce.

Health officials in the United States are not yet ready to blame the American outbreak to the leafy green. Still, they say, the Canadian finding has proved helpful and both outbreaks appear to have been caused by related strains of the bacteria, suggesting the possibility of a shared source.

“They’ve done a really thorough job there, and I think that gave us a really good clue to start with,” Dr. Matthew Wise, who oversees investigations into such outbreaks for the Centers for Disease Control and Prevention, said in a telephone interview on Friday.

In all, at least 58 people in both countries have been sickened, and two — one in California and one in Canada — have died. In the United States, the C.D.C. has so far linked at least 17 reports of illness in 13 states to the outbreak.