The 27-year-old woman is facing murder charges.
"I don't think we'll ever know whether or not we could have prevented this," said Nassau County Executive Tom Suozzi, saying that Brewer had "literally dozens and dozens and dozens" of contacts with people who tried to help her.
"We have to remember that this was a crime that was committed by an individual that apparently has some very serious mental health issues that are unpredictable," Suozzi said.
A funeral service was held Saturday for 6-year-old Jewell Ward, and her half brothers, Michael Demesyeux, 5, and Innocent Demesyeux, 18 months. Their bodies were carried into the chapel in a single casket.
Brewer remains in a hospital jail ward as she recovers from injuries she got when she jumped out a second-story window after calling 911 to report the killings.
The impoverished working-class community in Nassau County was devastated, said the Rev. Dr. William A. Watson, Jr., pastor of St. John's Baptist Church in Westbury, where the funeral was held.
"There is a sense of loss as opposed to anything else. Children have a way of touching your heart," he said earlier.
The clergyman said he intended to support Brewer. "We're not here to justify or judge. We are here to see if we can help. We are in the business of redemption," Watson said.
Only two days before the children's deaths on Feb. 24, case workers had gone to Brewer's home to investigate a report the youngsters might be in imminent danger from their mother.
They found no one home that Friday, and in an apparent bureaucratic slip-up a supervisor scheduled case workers to go back on Sunday, rather than the next day.
The supervisor, Eddie Arrendondo, has been suspended without pay as the county reviews more than 1,000 pending child welfare cases. The supervisor, who had 19 years on the job, is fighting the open-ended suspension through arbitration.
"We don't know what would have been found on Friday or Saturday," said Mary Curtis, a deputy county executive for health and human services. "I can't tell you what would have happened."
Public agencies had been aware of Brewer long before last weekend.
The county Child Protective Services agency had investigated 10 cases involving her in the past five years, three of which required follow-up. Brewer had been arrested seven times for various offenses since 2000 and had been referred to substance-abuse classes and parenting classes, as well as nonprofit assistance agencies, officials said.
However, whenever case workers investigated, they never determined that the situation required the children to be removed from the home. "The children seemed well cared for," Curtis said. They were clothed and fed, were going to school, and "the apartment was well taken care of," she said.
One time Brewer was found sleeping in her car once while the children were on a nearby playground, and another time she left them alone in the apartment to do laundry, officials said.
Brewer is due in court on Friday. Her court-appointed attorney did not immediately return a call seeking comment.
Nearly 1,500 children died from abuse or neglect in 2005, more than a fourth of them at the hands of their mothers, according to a federal government study.
Santa Clara University law professor Michelle Oberman, co-author of "Mothers Who Kill Their Children: Understanding the Acts of Moms From Susan Smith to the Prom Mom," said Brewer may have snapped under the pressure of raising her children as a single mother, dealing with disgruntled fathers and other disputes.
The father of the two boys, Innocent Desmesyeux, had a Family Court hearing scheduled with Brewer on the Monday after the killings to discuss a visitation schedule with his sons.
"The inclination is to say she's just evil," said Oberman, a defense witness for Andrea Yates, the Texas mother who in 2001 methodically drowned her five children in a bathtub.
"But this was a mom who was in over her head," she said of Brewer. "The most important thing in her life was her children; you'll hear that from any mother on any playground anywhere. Yet in this instance, she was sinking.
"The tragic irony is that at some deep level, this is about love and desperation. The cruel irony is she probably did love her children."