Lt. Gov. Anthony Brown in May 2013. (Astrid Riecken for The Washington Post)

The day before he was to testify about an emergency bill that would help those who were unable to sign up for health insurance through the state’s exchange, Maryland Lt. Gov. Anthony G. Brown (D) said Monday that he was kept in the dark about the severity of the problems that led to the calamitous debut of the state’s online health marketplace.

Brown, who is running for governor, told reporters that the state has made great strides in correcting many of the problems. He added that he shares in the responsibility for the troubled rollout. He again called for an investigation into what went wrong but said that at the moment the focus is on making sure the exchange is functioning properly.

As the highest-ranking elected official charged with implementing President Obama’s Affordable Care Act in Maryland, Brown plans to appear before two legislative committees in Annapolis on Tuesday afternoon to talk about emergency legislation that would provide retroactive health insurance to the hundreds — possibly thousands — of people who tried to enroll through the exchange but were stymied and left uncovered as the new year began.

The O’Malley administration plans to announce Tuesday that the four insurance carriers participating in the state’s exchange have agreed to provide coverage that is retroactive to Jan. 1 as long as residents sign up by Jan. 21 and pay their premiums for January and February, an administration official said Tuesday.

The official was not authorized to be quoted by name about the agreement with the four carriers, which was first reported by The Baltimore Sun.

Health officials plan to contact residents they have identified as having problems with the Web site and encourage them to sign up for coverage. The effort is expected to reduce the number of people who would register for the state-provided insurance that’s at the heart of the emergency legislation.

In addition to holding hearings on the emergency legislation, both committees also have scheduled broader briefings on the online marketplace. Members said they are eager to ask larger questions about how the exchange, which state leaders had said would be a national model, veered so far off course.

Del. Patrick L. McDonough (R-Baltimore County) said that his first question for Brown would likely be: “If you can’t manage the exchange, how are you going to manage the state as governor?”

Brown’s chief rival in the Democratic primary, Maryland Attorney General Douglas F. Gansler, pounced again Monday, calling a news conference to demand that Brown take responsibility for the exchange’s failings during the hearings.

“We want answers,” Gansler said. “What did he know and when? . . . In order to fix the problems, we have to know how it happened. . . . It’s easy to deflect blame.”

The Washington Post reported Sunday that more than a year before the exchange was launched, senior state officials failed to heed repeated warnings that the project was far behind deadline and lacked authoritative leadership. While Gov. Martin O’Malley and Brown were among the early and enthusiastic supporters of what became known as Obamacare, behind the scenes, the system went through three project managers, and contractors hired to build the site clashed endlessly. Meanwhile, state Secretary of Health and Mental Hygiene Joshua Sharfstein and others close to O’Malley and Brown received audit reports warning of trouble, according to distribution lists.

The day of the launch, the Web site crashed. Since then, residents have struggled with error messages, frozen accounts and incorrect information.

On Monday, Brown repeated to reporters that he and the governor were not made aware of potential problems until days before the Oct. 1 launch. Even then, the true depth of dysfunction was not conveyed, Brown said.

“What was presented at that time were characterized as minor glitches, some volume-related ­delays — nothing like what we saw on Oct. 1,” he said.

Brown said that his responsibility was to establish the “legislative framework for the exchange” and that its creation was entrusted to the exchange’s governing board, staff and contractors hired to create the Web site. Those people were supposed to provide complete and accurate reports of problems they were encountering — something that Brown said did not happen.

“I think that everyone involved in setting up the exchange bears responsibility — that includes me,” Brown said Monday morning after a ribbon-cutting at an Interstate 95 travel plaza north of Baltimore.

Later in the day, Gansler held the news conference at his campaign headquarters in Silver Spring and blamed Brown for the “debacle.”

Gansler spoke in a mocking tone about the lieutenant governor, who has a small staff and no prescribed constitutional duties. He also suggested that the task of setting up an online exchange should not have been difficult, relaying that he had registered his son in a lacrosse camp on the Internet the night before without any problems.

Soon after Gansler’s news conference ended, Brown’s campaign fired off a statement that said: “Once again, Doug Gansler sounds just like a Republican attacking health care reform. Instead of working with the Legislature and the Governor’s office to offer practical solutions, he is simply trying to score cheap political points to further his own political ambition.”

Brown stressed Monday that the administration has made a number of improvements to the exchange and will continue to do so. He repeatedly said that now is the time to get more Marylanders enrolled in health coverage before the March 31 deadline and that an investigation and blame-placing would have to wait.

More than 20,000 people had signed up for private plans as of Friday, and state officials have said they do not expect to hit their initial goal of 150,000 by the end of March. But the number of people enrolling in Medicaid plans has been higher than expected.

“Right now, we are focusing on the progress that we are making,” Brown said. “What we don’t want to do is to divert those resources, those people who are working on the fix to work on the investigation or the audit. But it will be done.”

Aaron C. Davis contributed to this report.