United Airlines and health officials are working together to get in touch with passengers who could have been exposed to COVID-19 when a male passenger died following a medical emergency on a recent flight.

The New York Times reports that the flight departed from Orlando, Florida, and was redirected to New Orleans when the man’s wife told emergency medical workers that he was infected with the virus. United then said on Friday that the wife was overheard telling the medical professionals that her husband had COVID symptoms, including loss of taste and smell.

A Twitter user who was allegedly a passenger on the flight wrote about the incident on Dec. 15, tweeting that “an entire plane had to watch him seize or have a heart attack none of us know which, and die.”

However, the airlines said that medical workers didn’t verify he had tested positive for the virus and still aren’t sure if he had it. When the flight arrived in New Orleans, United was told that the man had endured cardiac arrest. The CDC later contacted the airlines. “We are sharing requested information with the agency so they can work with local health officials to conduct outreach to any customer the C.D.C. believes may be at risk for possible exposure or infection,” United said.

On Friday, the CDC said it was “in the process of collecting information and proceeding according to our standard operating procedures to determine if further public health action is appropriate.”

Since the flight, the four flight attendants who responded to the emergency have started quarantining for 14 days after arriving at the plane’s final destination of Los Angeles.

United said that the man had filled out a “Ready-to-Fly” checklist where he said he hadn’t tested positive for COVID and didn’t have any symptoms. However, the man’s family later said he had been “feeling sick leading up to the day of travel,” per United. He also was immunocompromised with a history of high blood pressure and upper respiratory issues. United only learned that the man’s wife was worried he had exhibited COVID-19 symptoms after the plane landed in Los Angeles.

“The decision to continue on to Los Angeles was made after the medical professionals initially ruled the emergency as cardiac distress,” United said in a statement. “A change in aircraft was not warranted. Instead, passengers were given the option to deplane and take a later flight or continue on to Los Angeles. All passengers opted to continue.”

According to TMZ, the incident happened on Dec. 14, and now the CDC is attempting to contact all 179 passengers from the flight.