The Trump administration extended the deadline for U.S. consumers to enroll in health plans offered under the Affordable Care Act after the marketplace's website experienced technical problems, resulting in some Americans being locked out.

Enrollment for individual insurance coverage under the ACA offered by companies like Cigna Corp., Anthem Inc. and Oscar Insurance Corp., starting Jan. 1, 2020, was set to end Dec. 15. But the Centers for Medicare and Medicaid Services said it would extend that deadline to 3 a.m. ET on Dec. 18.

However, the agency waited well into the day on Dec. 16 before disclosing it would reopen the website for enrollment.

Just before 1 p.m. ET, CMS Administrator Seema Verma tweeted a statement from CMS and said her agency would again start taking enrollment at 3 p.m. ET out of an "abundance of caution."

Verma did not acknowledge the Dec. 15 technical problems with the website and noted only that some people "may have experienced issues" and "were asked to leave their name at the call center."

She said the call center and website had remained open for business on the last day of enrollment.

But Joshua Peck, co-founder of the advocacy group Get America Covered and the former chief marketing officer during the Obama administration for the healthcare enrollment website, said people were being blocked from logging in to enroll in ACA plans on Dec. 15.

He said the error people were seeing on the site, Healthcare.gov, was neither a "system down" nor a "waiting room" message, "leaving people trying to login bewildered that this is even a login page."

"When you call the listed number, the message states 'due to high call volume' leave your name and number," Peck tweeted on Dec. 15.

Sometimes the login page would load and other times it would not, he said.

"For context, right now a baseball stadium of people should be enrolling every hour," Peck tweeted. "These kind of technical issues could not be happening at a worse time."

Peck noted that there was a "growing chorus of people" on Twitter calling on CMS to extend the deadline, including former CMS Administrator Andy Slavitt, who led the agency during the last year of the Obama administration.

Democratic presidential candidates Sen. Elizabeth Warren, D-Mass., former Vice President Joe Biden and South Bend, Ind., Mayor Pete Buttigieg also weighed in on the website problems.

"Any delay or attempt to keep Americans from enrolling is completely unacceptable," Warren wrote.

"This is unacceptable," Buttigieg tweeted.

Biden suggested the downtime on the enrollment website was an attempt by the Trump administration to "sabotage" the ACA — former President Barack Obama's signature legislative achievement.

Some Democrats in Congress also said they were concerned.

"This is just another example of the administration's persistent efforts to undermine the ACA," Sen. Bob Casey, D-Pa., tweeted.

Rep. Frank Pallone, D-N.J., chairman of the Energy and Commerce Committee, said the website glitches were "inexcusable."

Pallone was among a group of Democratic House and Senate committee leaders who had urged Health and Human Services Secretary Alex Azar and Verma in a Nov. 21 letter to address the technical failures that inundated the ACA website on the first day of the six-week enrollment period.

As many as 100,000 fewer people may have signed up for coverage on the first day of open enrollment due to those problems, the lawmakers said.

They demanded to know if CMS had investigated the cause of the website errors, whether the problems had been fixed and if the agency had taken steps to ensure those issues did not occur again.

CMS has yet to disclose what led to the technical problems.

Once the website was reopened for business on Dec. 16, Slavitt noted that it still stated, "2020 open enrollment is over."

Later in the afternoon, CMS added a notice that the open enrollment had been extended until Dec. 18. "If you tried to enroll but didn't finish, there's still time," the agency stated.

Verma said consumers who had left their contact information with the call center on Dec. 15 would be contacted by a representative later in the week and did not need to make another attempt on the website to enroll in an ACA plan.