Montgomery County Executive Douglas M. Duncan (D) and members of the County Council said yesterday that it was unacceptable that they were not informed about last year's mistaken release of five inmates from the county's Rockville detention center.

"It came as a complete surprise to me," Duncan said. "It's not acceptable that I wasn't told. And it's certainly not acceptable that people were being released prematurely."

Duncan was responding to a report in yesterday's Washington Post about the increase last year in mistaken releases from the county's jail. Records show that five inmates were discharged by mistake last year, compared with one mistaken release in each of the prior two years.

Arthur M. Wallenstein, director of the county Department of Correction and Rehabilitation, said he had not viewed the errors with alarm because every inmate was recaptured within a few days and none was believed to have committed crimes while at large. He added that 22,000 inmates pass through his jail and the adjacent processing center each year.

But that explanation did not satisfy Duncan or council members yesterday. They said they view the mistakes as a serious safety matter.

"This is not a population that you want to take any chances with," said County Council President Michael L. Subin (D-At Large). "I think it's an inexcusable administrative error. . . . We need to find out what happened and make sure the proper measures are taken to make sure it doesn't happen again."

Council member Phil Andrews (D-Gaithersburg) said he will convene a special meeting of the Public Safety Committee on March 24 to get an explanation from correction officials.

"I have serious concerns," Andrews said.

The reasons for the foul-ups varied, according to Wallenstein. There were paperwork mistakes, miscalculated sentences and, in one case, a man released in place of a cousin with the same surname.

Although Wallenstein said in interviews that none of the cases warranted alarm, he sent an internal e-mail to top staff members last year calling one of the releases "a bona fide public safety emergency." And in a memorandum to members of the detention center's neighborhood advisory committee in January 2002, Warden William L. Smith said he considered an improper release "a very serious matter."

News of the errors comes as Montgomery prepares this month to transfer prisoners to its new $90 million detention center in Clarksburg. Council member Michael Knapp (D-Upcounty), whose district includes the new jail, said he believes the problems need to be resolved to bring peace of mind to those living nearby, and to those who live in Rockville, where all prisoners will be driven before they are released.