The U.S. Department of Agriculture should explain why it hasn’t been able to identify the source of contaminated meat that landed on the shelves of Hannaford supermarkets, said U.S. Rep. Chellie Pingree.

A week after Hannaford issued a recall of ground beef, the USDA would not say Wednesday whether it has made progress toward determining who is responsible for the contamination that led to an outbreak of salmonella.

“We have asked them what is holding up (the investigation),” said Pingree, a member of the House Agriculture Committee. “We get the same answer — that they are investigating the problem, but are having a difficult time tracking down the sources of the meat.”

Last week, the agency said Hannaford’s “limited records” were impeding the investigation, and that the USDA “is pursuing rulemaking to address the concern.”

USDA spokesman Neil Gaffney has not responded to repeated requests from The Portland Press Herald for information about what is lacking from Hannaford’s records or what new rules the agency is considering.

In reaction to the USDA’s reticence, Pingree criticized the agency Wednesday for not telling the public more about the weak record-keeping requirements that make it difficult to quickly track down the source of contaminated meat.

“It is good for the USDA to give more information to the public,” Pingree said. “A case like this helps the public understand why we need more record-keeping and tracking.”

Hannaford spokesman Michael Norton, who has said the grocery chain is compliant with industry record-keeping standards, wouldn’t speculate about what information would make its records more complete.

Pingree said the Scarborough-based grocery chain is not to blame for the difficulty of the investigation.

“They have been doing everything they are required to do,” she said.

Pingree said a “fault in our system” is the real hurdle. And it’s caused problems before.

Experts say Hannaford’s case is just the latest example of a foodborne illness investigation that’s been hindered by less-than-ideal record-keeping practices.

“It happens all the time,” said Bill Marler, a food safety lawyer from Seattle, Wash.

The problem is that stores often mix meat from different sources into a batch of ground beef, and they’re not required to record what meat was used.

“That is currently voluntary,” Pingree said. “This is a big system problem.”

Sometimes combing through records can be result in identifying a source of the contamination, but not always, said Marler.

“You might simply not be able to do it,” he said.

Pingree said the USDA is working on new rules that could be in place as soon as next year and would require stores to keep track “of every piece of beef that goes into a package of ground beef.”

It would help if more people were aware of the problem, she said, which is why the USDA should be making more of an example of the Hannaford investigation.

“If the USDA was more open with people about the fault in our system, we would be more likely to fix the system and have the political will to do it,” she said.