A CDC spokesperson said that the delay was because of a scheduling misunderstanding.

"On Saturday, February 22, at about 7 p.m., an FDA employee arrived at CDC Roybal campus in Atlanta, a day before what CDC understood to be his scheduled arrival time. Due to CDC security requirements, he was not allowed on campus that night," the spokesperson said. "On Sunday morning, February 23, as scheduled, CDC staff met the FDA employee and escorted him on campus, in full compliance with standard security processes required for all individuals whether they are federal employees or other visitors."

Stenzel later found evidence of lab contamination, which he reported to HHS officials and may have contributed to the coronavirus lab-test delays and other problems.

The CDC had spent days reassuring HHS leaders that the lab tests were imminent, even as delays prevented their delivery. The delays prevented many Americans, who didn't fit the CDC's strict criteria, from being tested for coronavirus. CDC initially limited testing to people who had recently traveled to China or had close contact with a confirmed case and were also symptomatic.

Health officials have reported more than 100 cases of coronavirus across the United States, with increasing evidence that the virus has been spreading undetected for weeks.

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CDC officials have acknowledged that the agency's lab tests were suffering flaws that prevented the health department from executing its plan to expand testing across the nation.

"Contamination is one possible explanation but there are others, and I can’t comment on what is an ongoing investigation," Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, told reporters on a Tuesday conference call.

HHS has begun an investigation into the possible contamination of coronavirus tests and is asking a team of non-CDC scientists to probe the lab-test defect.

Brianna Ehley contributed to this report.