An employee at the city-owned United Medical Center received several hundred thousand dollars in improper payments, the hospital’s board said Thursday.

The employee “inappropriately manipulated the IT/payroll system to be paid overtime hours” even though the person was not eligible to be paid overtime, according to a hospital statement. United Medical Center did not identify the employee.

An official in Mayor Vincent C. Gray’s administration, who was not authorized to speak publicly on the matter, said the overtime payments are in the range of $400,000, with additional bonus payments in the six-figure range also at issue.

The alleged manipulation, the official said, dated to before the city assumed control of the hospital in 2010. Management of United Medical Center was recently assumed by Huron Consulting, which was hired by the city on a two-year, $12.4 million contract to review and overhaul its operations.

The hospital has long been on shaky financial footing. In the 2012 fiscal year, it recorded a $322,000 loss after the District infused $7.7 million in cash and forgave a $6 million loan. Last month, the city gave the hospital an additional $11 million cash infusion to help cover unpaid bills.

David R. Small, who took over as chief executive on March 29, said the questionable overtime was discovered last week during a review of hospital operations.

Small the employee was placed on administrative leave on April 3 pending an investigation and dismissed Monday.

“We had sufficient information to move forward with a termination,” he said.

The matter has been referred to Inspector General Charles Willoughby and Attorney General Irvin B. Nathan for investigation, Small said. Ted Gest, a spokesman for Nathan, confirmed the office had received a referral but declined to comment further.

The hospital is undertaking a “root cause analysis” of what allowed the issue to go unnoticed for years, Small said.

“It appears all the right flags and checks and balances were okay,” he said. “It was someone manipulating the system to show there was not problem.”

But Small said he was “quite surprised” the issue wasn’t noticed in recent months as hospital management focused on reducing overtime costs. “I would have thought this might have materialized a little sooner,” he said.