In Iowa, 20 U.S. Food and Drug Administration inspectors are focusing on the two big egg farms involved in a massive recall, looking for commonalities among the windowless hen houses where the eggs were laid.
"They may have to look many places -- the chicken, and the eggs, at the chicken litter, the feed, the dirt, workers boots, processing equipment, fan blades many things could be a source of contamination," said William Hubbard, a former FDA associate commissioner.
Like detectives, investigators are tracing the bacteria back from the cluster of cases where numerous people got sick. The two affected farms have been identified as Hillandale Farms and Wright County Egg.
They first noticed a pattern in egg products used for a high school prom, a college graduation party and a catered breakfast in California.
"We went on several levels -- who was the caterer, who supplied the caterer and traced it back to this Wright County Egg in Iowa," said Jonathan Fielding, director for the Los Angeles County Department of Public Health.
A statement issued by Wright County Egg said it "has worked closely with the FDA through their review of our farms."
The company's owner, Jack DeCoster, is one of the country's top ten egg producers and has a long, troubled history with regulators.
But new federal rules requiring more testing on farms may help prevent future outbreaks.
"It will require egg producers to do some monitoring of their flocks and their environment for salmonella but you still need to do much more to make the egg supply completely safe," Hubbard said.
FDA officials said no further recalls are likely, which suggests investigators are confident they have honed in on the major sources of contamination in Iowa.
The eggs that are believed to be tainted were sold under the following brand names: Lucerne, Mountain Dairy, Sunshine, Hillandale, Trafficanda, Albertson, Farm Fresh, Shoreland, James Farms, Glenview, Mountain Dairy, Ralph's, Boomsma's, Lund, Dutch Farms, Kemps and Pacific Coast.
For a full list of egg brands and plant numbers affected by this recall, visit the Egg Safety Center at http://www.eggsafety.org.
Nearly 2,000 people have been sickened by salmonella linked to eggs in recent months, a number that is likely to rise.
ABC News' Emily Friedman, Brian Hartmen, Kim Carollo and Brad Martin contributed to this report.