An outbreak of food poisoning can fly like the wind, while the regulatory response still looks like a horse and buggy operation.

That’s what we take from the recent ground beef recall of meat sold by Hannaford supermarkets that sickened 19 people — including four in Maine — with a drug-resistant strain of salmonella.

Three weeks after the meat was pulled from the shelves, we still don’t know the sources of the infection, which is key information for people who are supposed to have confidence in the food supply.

U.S. Rep. Chellie Pingree is pressing the Department of Agriculture to be more forthcoming about the kinds of investigations they conduct and the types of results we should be able to expect. Among Pingree’s questions is how often the federal government probes come up empty and no source is discovered for the outbreak.

State disease control officials in Maine and New York were able to follow the clues from sick individuals to Hannaford.

But federal authorities have not been able to shed much light on what happened before the meat was sold, with one official blaming the supermarket chain’s practice of mixing beef from different sources when it grinds it into hamburger.

Hannaford announced this week that it would no longer mix beef before it is ground, even though that is legal, making it easier to trace future outbreaks. But if mixing meat from different sources is risky because it slows down the ability to track the source of contamination, then the agency should be ending the practice for everyone.

Changes in the way meat is produced and processed make the food supply more vulnerable than ever to outbreaks of disease. Slaughterhouses and processing plants are centralized, allowing an infection to spread in a way that would have been impossible when those procedures were done locally.

Because meat is likely to cross state lines multiple times before it is put on a dinner table, regulating it is a federal responsibility.

To do that well requires thorough and transparent investigations and timely communication with the public, something that has been missing so far from this salmonella outbreak.