(CNN) The likely source of a deadly mold outbreak that started in 2014 at University of Pittsburgh Medical Center hospitals was the hospitals' linen provider, according to findings shared in emails exclusively obtained by CNN.

UPMC informed the Allegheny County Health Department, Pennsylvania Department of Health, and U.S. Centers for Disease Control and Prevention that its independent testing led UPMC officials to believe the linens were the likely source of the outbreak, according to the emails.

The new emails reveal additional testing seeking to pinpoint the source of the outbreak.

An email sent May 5, 2016, by Jeff Miller, a field epidemiologist for the CDC, said that environmental samples were taken on February 21, 22 and April 21, 2016, at UPMC Presbyterian and Montefiore hospitals and Paris Co. laundry facilities. The testing was requested by UPMC and conducted by an independent contractor.

A whole genome sequencing report by an independent laboratory based on these specimens "[suggests] that fungi from Paris have been brought into the [Presbyterian University Hospital] laundry," according to the report provided in the email.

"It is looking more likely than ever, that the linen vendor is the more likely potential source of mucor as you can see by the reports," said an email sent by Allegheny County Health Department Chief Epidemiologist LuAnn Brink on June 21, 2016, in which she shared "notes from UPMC." It is unknown who from UPMC sent Brink the notes. Mucor is a species of mucormycosis , which are fungal spores commonly found outdoors. However, they can cause a rare and sometimes fatal fungal infection in patients who are immunocompromised, according to the CDC.

"As can often be the case, a singular email can often be taken out of context. The email was not reflective of my thoughts or knowledge at any time during this investigation. Instead, it was a compilation of information that was used to brief Health Department staff on all the potential sources of discussion at the time the email was sent," Brink said in a statement to CNN about the email.

A whole genome sequencing report compares environmental samples from which the DNA has been extracted and sequenced to show the fingerprint of the sample at a high resolution, according to Dr. Barun Mathema, an assistant professor of epidemiology at Columbia's Mailman School of Public Health who CNN contacted independently on Monday.

"Nobody is a 100% sure, if you read carefully, there will always be a bit of a wiggle room caveat," said Mathema, who was not involved in testing the samples from Pittsburgh. In general, it is rare that epidemiological testing comes to an absolutely conclusive result because of variables that come into play when epidemiologists attempt to replicate the exact environment in which the case occurred.

Additional testing

This isn't the first report to link the mold outbreak to hospital linens. In a separate, UPMC-contracted internal report , testing conducted on February 1, 2016 by hospital environmental specialists Andrew Streifel and Michael Buck found a heavy buildup of lint and mold near the Paris Co. linen facility vent through which unfiltered air dried the linens.

When Streifel and Buck went to the hospitals and tested linen from Paris Co., a cart of wet sheets delivered to the Montefiore laundry storage area bore "heavy fungal growth of Mucor and rhizopus," according to their report, which was not made public until early 2017.

"Our hospitals are safe, and our ongoing monitoring and testing show no evidence of a mold outbreak. We and the nation's top health regulators have found no definitive or unifying cause of the previous infections, which are known to occur on occasion at most major medical centers," UPMC spokeswoman Allison Hydzik said in a statement to CNN when asked to respond to the contents of these emails.

UPMC "will address specific allegations in court and not in the media," Hydzik said.

The Allegheny County Health Department provided CNN with the emails, which were incorrectly redacted to reveal more information than intended. The emails were provided in response to an open records request under the state's Right to Know Law. The county records department withheld several other emails on the grounds of privacy for patients' information.

"While we were made aware of all steps that UPMC was taking to identify potential sources, including looking at linens, there was no identification of the linens as the source of the outbreak. Similarly, as has already been stated by the department, ACHD was an observer in this process, not the regulatory agency looking into this," Allegheny County Health Department spokeswoman Melissa Wade said in a statement to CNN when asked about the emails.

"Our products are safe. We have nothing to hide. We have followed and continue to follow protocols of the Healthcare Laundry Accreditation Council by providing hygienically clean linens to our customers," Paris Co. CEO Dave Stern said in a statement to CNN when asked about the content of the emails.

CDC and state officials respond

In a phone interview this week, Sharon Watkins, director of the bureau of epidemiology for the Pennsylvania Department of Health said the mold investigation is continuing. At no time has the state or the CDC determined that linens were the most likely cause of the UPMC mold outbreak, she said.

"I don't know why anyone would say that," Watkins said, referring to the email that said "the linen vendor is the more likely potential source of mucor."

UPMC is required to provide the Pennsylvania Department of Health with regular updates concerning the investigation, Pennsylvania Department of Health spokeswoman April Hutcheson said.

Since May 2016, the Pennsylvania health department has been "in constant consultation with the CDC on this issue, including epidemiological evaluation of new information, a site visit, and current review of system-wide UPMC data," Hutcheson told CNN in a statement when asked about the emails.

"There is no evidence to indicate an ongoing outbreak at this time; however, the state health department is in the process of reviewing additional information as part of the ongoing investigation."

The UPMC hospital system mold outbreak began in October 2014. By September 2015, four patients had died of fungal infections at UPMC Presbyterian and Montefiore hospitals. By that time, the transplant ICU at Presbyterian had temporarily closed.

In September 2015 , while the Presbyterian transplant ICU was closed, the CDC investigated possible sources of the mold outbreak. The results of the investigation were published in the May 2016 Morbidity and Mortality Weekly Report . Investigators said that the fatal infections were not attributed to the mold-covered linens, but rather to ventilation that may have allowed dust and mold spores to enter the hospital rooms.

"There was no epidemiology pointing to linens being the source," Skinner said in response to this article. The outbreak would have involved more skin-related cases if laundry were the source, according to Skinner.

The CDC maintains that conclusion despite what the newly uncovered emails say, Skinner said. The CDC findings were not addressed in the recently released emails.

In May 2016, Pennsylvania Governor Tom Wolf again requested the CDC consult with the state health department and UPMC because a fifth fungal infection recipient had been identified in the Presbyterian ICU, CDC spokesperson Tom Skinner told CNN when asked about the content of the emails. Wolf's press secretary, J.J. Abbott, confirmed the request in an email to CNN.

That additional patient was Daniel Krieg, a kidney transplant patient who died in July 2016. UPMC medical reports show Krieg had fungal pneumonia and that fungal-infected sections of his left lung were removed. A CDC representative returned to UPMC on June 22, 2016, with officials from the state and county departments of health to discuss the outbreak and Krieg's case, Skinner told CNN on Monday.

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An agenda in the newly revealed emails said health officials were to discuss laundry processes at Presbyterian, Montefiore, and Shadyside hospitals at the June 22, 2016 CDC visit.

The day before that visit, an June 21, 2016, email sent by Brink also said that UPMC was working with the linen vendor to "improve their product." The email also said the hospital system was "aggressively pursuing" an alternative linen solution for all transplant patients in the ICU at Presbyterian hospital.

UPMC said in a January 2016 statement that high-risk transplant patients would receive linens with a lower bacterial count going forward. Hospital linens are required to be " hygienically clean ," according to the Healthcare Laundry Accreditation Council. (PDF) That means linens are not sterile, but in a clean state, free of pathogens in sufficient numbers to minimize risk of infection, and the clean textiles are not inadvertently contaminated before use, according to the CDC.

The Textile Rental Services Association of America Inc. issues "hygienically clean" certifications to healthcare laundries that seek it. The certification requires quarterly ongoing microbial testing and a supplemental second inspection during a facility's three-year certification period, according to TRSA President and CEO Joseph Ricci. Paris Co. does not have the hygienically clean certification, according to Ricci. "Paris is an active member of TRSA, and serves on Committees and other roles but is not certified," Ricci said in a statement to CNN.

"Many hospital laundries have received the Hygienically Clean Healthcare certification, which is recognized as the highest standard of certification in healthcare linen safety and cleanliness, and is the only healthcare standard in North America that requires initial and ongoing quarterly microbiological testing based on internationally recognized protocols and standards," Ricci said in a statement to CNN in response to this article.

"Hygienically Clean Healthcare tests for molds and yeasts, and after more than 3,000 microbial tests of healthcare linens and garments, there has never been any evidence of Rhizopus or other dangerous molds," Ricci said, referring to the association's testing from many facilities.

"Despite the lack of a definitive source, UPMC still went above and beyond state and federal recommendations in order to implement changes to protect our patients. One of the many changes includes the provision of specially treated bioburden-reduced linens to our highest risk transplant patients," UPMC spokeswoman Hydzik told CNN previously.