But officials from the Food and Drug Administration weren't saying yesterday what they've concluded or whether their work is done. Previously, the FDA said it could spend two weeks or more in Mexico.
The team of investigators includes specialists from the FDA and Centers for Disease Control and Prevention as well as Mexican agricultural and food-safety investigators.
Investigators have indicated they may never be able to pinpoint the precise cause of the outbreak.
Dr. Javier Trujillo, undersecretary for food safety and quality in Mexico's Ministry of Agriculture, said no direct evidence of hepatitis A virus was found during the inspections, though one firm did have water-quality problems.
Trujillo said investigators visited facilities owned by four companies that the FDA identified as sources of green onions linked to hepatitis A outbreaks this fall in Tennessee, Georgia and the Beaver Valley Mall Chi-Chi's in Center. Last night, he said, the FDA investigators left Mexico without finding further problems.
While FDA officials could not be reached for comment yesterday, an import alert issued by the agency last month -- and posted on its Web site yesterday -- provided the most detailed written explanation thus far for why the agency has stopped green onion exports from three of the Mexican firms.
"No known non-human sources of the virus exist," the agency said. "This fact, in conjunction with epidemiological evidence, leads FDA to believe that the implicated green onions were contaminated as a result of unsanitary conditions in the production or packing facilities, e.g., poor worker hygiene, inadequate worker sanitation facilities, and/or contaminated water supply."
But how and where the green onions were contaminated remained unclear.
Trujillo said that investigators found one firm was rinsing onions with water that did not meet water quality requirements.
Dos M Sales, one of the four firms implicated by the FDA, uses a packing house in La Rumorosa, an area west of Mexicali. It's a region with little water, and the packing house was using water from a natural reservoir, he said. The water did not meet standards for rinsing produce, he said.
Trujillo emphasized, however, that no evidence of hepatitis A was found in the La Rumorosa packing house, nor did he expect it to be.
U.S. investigators have said that matching the strains of the outbreak viruses to particular human sources is virtually impossible, due to the limits of medical records in Mexico. Though the virus can live for a month in dried feces and longer in ice, investigators have expressed pessimism about finding samples of either that would date to late summer and early fall, when the implicated scallions would have been harvested.
What investigators are looking for is basically circumstantial evidence, Trujillo said -- conditions under which the virus could be harbored and transmitted to produce.
U.S. officials said yesterday that an announcement will be made next week concerning a new infectious disease warning system along the Mexican border. On Friday, U.S. Department of Health and Human Services Secretary Tommy G. Thompson will make the announcement during the annual U.S.-Mexico Border Health Commission meeting in Saltillo, near Monterrey.
It's unclear, however, whether any such system could detect hepatitis A virus.
Thompson's appearance with Mexican health Secretary Julio Jose Frenk Mora will also call attention to new regulations designed to improve food safety, according to an HHS spokesman. Companies exporting food to the United States are being asked to give prior notice to the FDA about when and where their shipments are crossing the border.
Shippers and distributors who move food will also be required to keep better records about the origin and destination of products. The proposed rule, which should be completed by Dec. 12, would allow the FDA to determine more quickly the potential sources and distribution chain for contaminated foods.
Locally, the fallout from the Beaver County outbreak continued, with the Allegheny County Health Department reporting that nearly 600 people have opted for hepatitis A vaccinations since mid-November. Usually, the Health Department would expect to vaccinate no more than 100 people in the same time period, said spokesman Guillermo Cole.
"The demand has increased five-fold, close to six-fold," Cole said, saying demand is being driven both by the outbreak and a Health Department discount on the vaccine.
The final tally of blood donations pulled because of the outbreak stands at roughly 55, blood bank officials said yesterday.
Central Blood Bank and the Greater Alleghenies chapter of the American Red Cross asked last month that blood donors who ate at Chi-Chi's in September and October contact the blood banks so their donations could be pulled. There's a slight chance that the hepatitis A virus can be transmitted through a blood donation.
Deborah Ervin, spokeswoman from Central Blood Bank, said about seven potential donors have been turned away because they ate at Chi-Chi's during the time of the outbreak. Both blood banks have had to cancel blood drives in Beaver County in recent weeks due to hepatitis A concerns.
"A couple sponsors in Beaver County decided it was best to cancel our visit because they thought a significant number of perspective donors had eaten at Chi-Chi's and couldn't donate," said Marianne Spampinato, spokeswoman for the local chapter of the Red Cross.