ANNAPOLIS, SEPT. 25 -- Maryland corrections officials conceded today that an error led to the early release of a Harford County man who is charged in three killings committed during a time he should have been in prison.

During a legislative hearing today, an attorney for the Division of Correction said the state could be sued by relatives of the victims if John F. Thanos's release 18 months early is found to have been caused by negligence.

Until today, corrections officials had said only that they were investigating whether there had been proper application of a complex set of guidelines used in determining how much time to deduct from Thanos's sentence for good behavior. "Good time" is awarded as a method of maintaining discipline in prisons and to ease crowding.

But Bishop L. Robinson, secretary of public safety and correctional services, told lawmakers this afternoon that Thanos was "erroneously credited" with 543 days of good conduct.

"This speaks to me of some form of negligence," said Del. Charles J. Ryan (D-Prince George's), chairman of the legislative committee that held a hearing on the Thanos case. "He may have killed three people and he should have been behind bars."

Robinson told the House Appropriations Committee that a search of records found no other inmates who had been set free mistakenly.

Thanos, 41, of Joppa, was released April 5 from the Eastern Correctional Institution in Somerset County, where he had served four years of a seven-year sentence for a Harford County robbery. Had he not been mistakenly credited with "good time" earned during a previous prison sentence for rape, he would have remained in custody until September 1991.

Since his release, Thanos has been charged with two murders, an attempted murder and two robberies in Baltimore County and the Eastern Shore between Aug. 29 and Sept. 4, when he was arrested. The attempted murder charge could to be upgraded to murder because a youth shot Labor Day died Sept. 9. Thanos is being held without bail at the Worcester County Detention Center.

Robinson said today that corrections officials have a "monumental problem" in calculating good time under incentive programs linked to behavior, work, education and prison crowding. The difficulty is compounded, he said, when overlapping or concurrent sentences are involved.

"It's a nightmare, because most of the inmates have multiple sentences," Robinson said. Without early-release programs, the state would have to build hundreds of costly additional cells, he said.

On the Thanos case, Robinson declined to identify the corrections employees involved, saying that an investigation and disciplinary proceedings are ongoing.

Alan D. Eason, an assistant attorney general for the Division of Correction, said the state could be held liable for negligent acts of employees but had never faced a circumstance similar to the Thanos release. Another government lawyer said that claims against the state may be limited to $100,000 for each incident.