A third person has died following an outbreak of respiratory illness at a Fairfax County assisted-living facility that began more than two weeks ago, county health officials said Wednesday.

The outbreak at Greenspring Village in Springfield also spread to the unit’s staff, affecting 19 employees, Fairfax County Health Department officials said.

At a news conference Wednesday at the agency’s headquarters, Benjamin Schwartz, director of epidemiology and population health at the Fairfax County Health Department, said tests, including those conducted on 17 samples by the federal Centers for Disease Control and Prevention, have failed to identify a likely cause. Tests for Legionnaires’ disease have also come up negative. Officials tested for a range of common virus- or bacteria-borne respiratory illnesses.

Schwartz also reassured the public yet again that the company that operates Greenspring has been keeping the agency apprised of changes and carrying out stepped-up sanitary and cleaning measures to halt the infection, including limiting visitors, social gatherings, group dining and new admissions. The outbreak has been confined so far to Greenspring’s assisted-living and skilled-nursing unit, which has 263 residents. People who have become ill have been quarantined in their rooms.

But some families have not been satisfied with Greenspring’s handling of the outbreak. Terese Brewster, whose 89-year-old mother is a resident of Greenspring’s assisted-living unit, criticized the company for failing to notify families about the outbreak and keep them informed about developments.

Brewster, who lives in Arlington, said the company issued a written notice on July 10 about the outbreak to her mother, who has dementia. But Brewster said the company didn’t inform her or an adult sister who lives nearby until accounts of the outbreak appeared in the media. Since then, in phone calls to on-site staff, Brewster has received reassurances but few details about what’s happening.

“I’m just shocked. I just keep thinking if it was your parent, what would you expect?” Brewster said Wednesday.

The complex is operated by Erickson Living, which serves about 24,000 people in a network of campuses for older people in 11 states.

Courtney Benhoff, an Erickson Living spokeswoman, defended the company’s response.

“Greenspring has been consistent and transparent in its communications to all residents, staff, family members and other stakeholders throughout," she said in an email, adding that the staff has kept the community informed through telephone calls, “in-house broadcast updates” and face-to-face conversations. She also said the Springfield location’s staff appears to be containing the infection, with no new cases reported in the past 24 hours and no new hospitalizations. Most of those residents who had been hospitalized have returned to Greenspring, and there have been no new illnesses among employees since Friday, Benhoff said.

The notice that went out on July 10 from Donna L. Epps, an administrator at Greenspring, said several residents had been having symptoms of respiratory illness, including fever, coughing and body aches. Epps’s notice, which says the symptoms recede in about five to seven days with treatment but have caused pneumonia, also announced limits on visitors, enhanced sanitation measures and other steps.

“You may have seen media reports referring to these illnesses as an ‘outbreak’, which we understand can be alarming,” Epps wrote in a follow-up on Friday that said 50 people had been sickened. “In this letter, I want to provide you with clarity, transparency and the reassurance that we are taking all of the proper precautions to safeguard the health of this community and the well-being of all who live and work here.”

Late Tuesday, the Health Department gave an updated tally, saying 63 people in the assisted-living and skilled-nursing unit have become sick. The agency said there have been no new hospitalizations since 23 people were admitted after the outbreak began June 30. The agency was alerted to the outbreak on July 8.