As more illnesses are linked to ground beef that was sold at Hannaford Supermarkets, the company says it has changed its practices for grinding meat, and U.S. Rep. Chellie Pingree is pressing federal officials to provide more information about their investigation.

The U.S. Centers for Disease Control and Prevention released new information Wednesday about a salmonella outbreak that first was reported Dec. 15, when Scarborough-based Hannaford recalled ground beef with sell-by dates of Dec. 17 or earlier.

The CDC says the number of reported cases of the rare strain of salmonella typhimurium is now 19 — up from 14 cases on Dec. 15 and last week’s total of 18. The number of cases in Maine hasn’t risen from the four initially reported.

The other 15 cases have been reported in six states. New Hampshire has had the most, with six, followed by five in New York. Single cases have been reported in Vermont, Massachusetts, Kentucky and Hawaii.

The USDA is investigating the source of the contamination. In a statement Thursday, its Food Safety and Inspection Service said it “has not been able to trace the source of the outbreak beyond Hannaford because of inadequate records and grinding practices at the store chain.”

The statement said, “As our investigation continues, the retail chain indicated they intend to re-examine their grinding practices and make appropriate changes.”

Hannaford, which has 179 stores in the Northeast, has contended that it complies with industry standards for record-keeping.

On Thursday night, Michael Norton, a spokesman for Hannaford, said, “We’re doing more than what’s required and were before.”

Norton said Hannaford has “simplified” its grinding practices so that its records are more clear about the source of ground beef.

Previously, he said, some packages of ground beef contained meat from more than one source, which is allowed by the USDA. Hannaford stopped that practice on Dec. 15.

The CDC said 14 of the infected people reported eating ground beef in the week before their illnesses began. Twelve of them reported buying that beef from Hannaford stores in the period from Oct. 12 to Dec. 10.

The CDC knows that at least seven of those people were hospitalized. None have died.

The recall covers ground beef that carries Hannaford, Nature’s Place and Taste of Inspirations labels. The company is offering full refunds for any ground beef purchased within the scope of the recall, regardless of whether it is in the package.

Today, Pingree will send a letter to U.S. Secretary of Agriculture Tom Vilsack telling him that the “public has a right to know” more about what the USDA is doing to track down the source of the salmonella infection.

Willy Ritch, Pingree’s spokesman, said Thursday night that, “Consumers need to feel confident that the food we buy is safe, and when a problem like this crops up they need to know that authorities can trace it back to its source.”

Pingree, a member of the House Agriculture Committee, isn’t satisfied with all of the answers the USDA is giving about the investigation.

“I understand that certain information cannot be released until the USDA has completed its investigation,” Pingree wrote in the letter to Vilsack, which was provided Thursday night to MaineToday Media. “However, (the Food Safety and Inspection Service) is currently conducting an investigation into the origin of the ground beef that has been linked to the salmonella cases, and I do think the public has a right to know the steps that USDA is taking to trace the origins of any contaminated beef …”

Pingree told Vilsack in the letter that she has several questions about the investigation:

“What steps do USDA and FSIS take to determine the source of contaminated meat? How many investigations into contaminated meat did USDA conduct in the last year and the last decade? Of those investigations, how many involved ground beef? What is the average length of an FSIS investigation into the sale of contaminated meat?”

Pingree also asked: “How many times has FSIS been unable to determine the supplier of contaminated meat?” And, “Is Hannaford Supermarkets required by the USDA to keep records of which suppliers provide beef for their re-sale?”

Pingree said in the letter that she supports USDA rules that “require retailers to keep records that will allow quick determination of the source of any contaminated meat that is sold.”

Ritch said that if those rules aren’t adequate, Pingree will “push the (Obama) administration to make some changes.”

Staff Writer Leslie Bridgers and MaineToday Media Washington Bureau Chief Jonathan Riskind contributed to this report.