Allegheny County officials defended their 911 system yesterday while announcing plans for an outside review in the wake of three Pittsburgh police officers' deaths following a mishandled call.

County Executive Dan Onorato and Chief of Emergency Services Robert A. Full said "human error" was likely to blame for a failure in an otherwise top-notch system whose training exceeds state-mandated minimums.

"We don't believe this was a systematic issue by any stretch of the imagination," Mr. Full said during a news conference.

However, Mr. Onorato said, "The facts in this case are not disputed. There was a mistake made, a horrible mistake that was made."

County officials have said a part-time 911 operator failed to follow protocols for a domestic disturbance call April 4 in Stanton Heights by not passing information to a police dispatcher that there were weapons at 1016 Fairfield St.

Police say a heavily armed Richard A. Poplawski ambushed officers Paul J. Sciullo II, Stephen J. Mayhle and Eric G. Kelly, fatally shooting them and wounding Officer Timothy McManaway before surrendering.

Mr. Poplawski's mother told a 911 call-taker that her son had "legal" guns, but the operator did not press for more details and notified a police dispatcher that there were "no weapons."

The dispatcher did not relay any information about weapons to the officers.

Mr. Full has informally requested a review by the Pennsylvania Emergency Management Agency. PEMA spokeswoman Ruth A. Myers confirmed discussions but said a formal request has not been made.

PEMA hopes to have staff at the Allegheny County 911 center within two weeks to review training and how employees work calls as well as to assess the handling of the call in Stanton Heights.

"Whoever goes out will listen to the 911 call, but they will also be reviewing other calls as well," Ms. Myers said. "We're not going out there to listen to this one call to point the finger at someone. We're going to look at the entire department."

Since the incident, the county has instructed 911 operators to refer to a checklist when handling certain calls to make sure questions about weapons and guns on the premises are asked and then documented for police dispatchers.

County officials also said yesterday they were in contact with the National Emergency Number Association, a trade group, to conduct a performance review.

However, Michael W. Weaver, the group's Pennsylvania chapter president who has been in touch with Allegheny County, said his organization does not do such reviews at the state level.

The national group might come in to recommend best practices if a PEMA audit uncovers problems, he said, but as of last evening, the national association said it had not been contacted.

Rick Grejda, business agent for Local 668 of the Service Employees International Union, which represents 911 operators, embraced a PEMA audit.

"We can tell the public that this is a one-in-a-million error, that this happened one time and we took 1.5 million other calls last year and this never occurred," Mr. Grejda, a former dispatcher, said. "But in order to regain the public trust, we need the professionals. We need the people who have oversight authority in this matter."

The operator who handled the call from Margaret Poplawski remains on administrative leave with pay and is receiving counseling.

Officials said yesterday she had the state-mandated 320 hours of training, had been told April 2 that she was accepted for a full-time position, and received high marks from her supervisors.

"This individual had a bright future before them," Mr. Full said. "This wasn't a problem child by any stretch of the imagination."