The U.S. missed several crucial opportunities to slow the spread of COVID-19, which has infected more than a million people and killed nearly 70,000, according to a report released by the U.S. Centers for Disease Control and Prevention (CDC) on Friday.

The report, penned by CDC Principal Deputy Director Anne Schuchat, listed four factors that had accelerated the spread of the coronavirus.

"I think in retrospect, taking action earlier could have delayed further amplification [of the U.S. outbreak], or delayed the speed of it," Schuchat stressed later in an interview with the Associated Press.

The four factors are:

1. Travelers carrying the virus keep flooding into the U.S. Though the government banned entrance from China very early, it didn't work for Europeans who caught the virus in home country and then traveled to America. It's until March 11 that the government shut down travels from Europe.

2. Large gatherings like festivals, meetings and other events. There was Mardi Gras celebrations in Louisiana in late February, which attracted more than a million attendees. If the government had told the people to stay at home, the amplification of the COVID-19 spread would have been much weaker.

3. Workplace spread like hospitals and companies. Patients with fever and cough went to hospitals and may have spread the disease to health workers, who often work at multiple health facilities and unknowingly helped the spread of the coronavirus.

4. Patients without symptoms who cannot be detected without thorough testing. The government issued limitations on who can get the test, which was too strict at first and many people with symptoms were not sure if they had COVID-19 or other diseases.

"During a 3-week period in late February to early March, the number of U.S. COVID-19 cases increased more than 1,000-fold," the report noted.