A 76-page grand jury report additionally characterized the department as having "lax, unprofessional, and inadequate procedures and practices," and said the office "proved to be more of an obstacle than a facilitator for those who had sought to have dependents removed from coverage."
No individual employee was singled out for rebuke.
Human Resources Director Meredith Dolan could not be reached for comment Friday. An employee at the office said no one else was available to comment.
Heckler's investigation began last June after a county audit claimed that 192 people were mistakenly listed as dependents and thus were continuing to have their plan funded by the county.
The investigation found that 153 dependents were eligible for the plan at one point, Heckler said, and that many of the 39 employees with ineligible dependents had acted properly to try to remove them from the plan - if a child aged out, for example, or the employee divorced a spouse.
The Human Resources Department, Heckler concluded, simply failed to follow through - an example of incompetence but not criminality.
County Controller Raymond McHugh, whose office collaborated with Heckler's on the investigation, said the Human Resources Department was "a county department that has lost its way," and that he was "angry that the incompetence of this department" cost taxpayers nearly a half-million dollars.
The lost costs were the result of premium payments, the report said. None of the 39 improper dependents - about 5 percent of all dependents on the county plan at the time - had medical treatment paid for by the county, according to the report.
County Commissioner Robert G. Loughery said that procedures had been enacted to try to prevent recurrences. Last year, for example, the county began making employees submit annual forms that list their dependents.
Loughery vowed further action in the wake of the report, and did not rule out the possibility of firing Human Resources staff. He declined to offer specifics, saying officials need to read the grand jury's report before making any decisions.
"We now know we have some problems," he said. "We need to address them."