According to the USDA, Boar's Head factories across the country are now the subject of criminal investigations

Boar's Head's nationwide processing plants are now part of an ongoing law enforcement investigation, the U.S. Department of Agriculture announced Thursday, following a fatal outbreak traced to some of the company's since-recalled sausage products.

At least 59 hospitalizations and 10 deaths have been linked to a strain of listeria traced to Boar's Head products distributed by a now-locked plant the company operated in Virginia.

“We take public health and corporate accountability issues very seriously, and an investigation into this matter is ongoing,” a spokesperson for USDA’s Food Safety and Inspection Service (FSIS) said in a statement.

The Boar's Head plant in Virginia has been written dozens of times of inspectors for violations, CBS News first reported in August based on records obtained through a Freedom of Information Act request. Violations reported by agriculture officials at the plant included mold and insects found throughout the site.

FSIS disclosed the existence of the investigation Thursday in a letter denying a CBS News request for records about Boar's Head's other plants in Michigan, Arkansas and Indiana.

Because the records are being “compiled for law enforcement purposes,” FSIS officials said their release could “impair the government's ability to further control and shape the investigation.”

“In light of the ongoing investigation into this matter, FSIS is withholding 93 pages in its entirety,” they wrote.

The revelation comes on the same day as a letter from Senator Richard Blumenthal and Representative Rosa DeLauro, both Connecticut Democrats, to the USDA and the Justice Department urging them to consider criminal charges against the company.

“The time for action is long overdue, and we call on your authorities to work together to achieve immediate justice for affected consumers and prevent this from happening again,” they wrote.

It is unclear whether the law enforcement investigation is a criminal or civil investigation. An FSIS spokesperson said they could only provide limited details about an ongoing investigation.

A spokesman for Boar's Head declined to comment, saying the company does not comment on legal matters.

In addition to an investigation of “Boar's Head facilities across the country,” the agency is also conducting a “top-down” survey to look for public health lessons in the wake of the outbreak, according to the FSIS statement.

“We also place particular focus on government inspection models, including where policy changes are needed to prevent oversight gaps,” the statement said.

As CBS News previously reported, the USDA had outsourced most of its duties at the Boar's Head plant in Virginia to state inspectors under a decades-old agreement governing federal oversight of rural facilities.

“Despite the closure, the FSIS investigation into the factors that contributed to this outbreak, including but not limited to an in-depth investigation of other Boar's Head facilities, continues in the interest of best protecting the public health,” the spokesperson said.