BOSTON — Embattled Massachusetts Department of Children and Families Commissioner Olga Roche resigned Tuesday under mounting pressure following the deaths of three children.

The move comes after calls from top Democratic lawmakers — including Massachusetts House Speaker Robert DeLeo and Senate President Therese Murray — for Roche to be replaced as head of the child welfare agency.

“I have accepted her resignation because I believe it is not possible for the agency to move forward in this environment with her at the helm,” said John Polanowicz, secretary of the state’s Executive Office of Health and Human Services, which oversees the DCF.

Authorities say the family of a 2-week old Fitchburg baby who died over the weekend was being monitored by state child welfare officials.

In the second case, a DCF spokeswoman says a fax from Grafton police about possible harm to a 4-week-old who later died was misplaced for six days.

The body of a third child, 5-year-old Jeremiah Oliver of Fitchburg, was found off a highway in central Massachusetts this month. Jeremiah’s family was also under DCF supervision at the time of his disappearance.

Gov. Deval Patrick, who had defended DCF Commissioner Olga Roche, said Monday he no longer had confidence in the department and said Roche “has some questions to answer for me.”

Patrick said Tuesday it was clear that Roche had the expertise to do her job but no longer the support of the public or her staff. “We can’t wait until the end of the year. We can’t wait for a new governor,” DeLeo said Monday. “We have to take action now.”

Murray issued a statement saying “in combination with Commissioner Roche’s resignation, we need to make sure we are bringing someone in to turn this agency around.”

Patrick said he was as upset as anyone about the deaths.

“I don’t have confidence at this point in the agency and I’m very worried about the agency,” Patrick said Monday. “My confidence in the whole organization has been rattled.”

Patrick said his top concern is ensuring the welfare of children under DCF care.