Two inmates escaped from the Fairfax County Adult Detention Center in Fairfax City last night, setting off a search aided by dogs and a helicopter, county police said.

The escape was undetected, and the men were not missed until about 10 p.m. during a head count at the jail at 10520 Judicial Dr., police said. They said the men were last seen during a similar count a half-hour before.

Jim Vickery, chief county sheriff's deputy, said he believes that the men, who wore gray sweat pants and white T-shirts, escaped through a hole in the wall of the cell assigned to one of the two.

Authorities said the escape was apparently the first from the 900-inmate jail, which opened at the county's administrative center in the late 1970s.

Vickery said the hole, barely big enough to squeeze through, was apparently made earlier in the day, possibly by smashing a stone tabletop against the wall. Jackhammers at a nearby construction site may have masked the noise, he said.

According to Vickery, the hole in the wall leads directly to the outside of the jail. He said he would call for improvement in construction of the walls.

The escapees were identified as Willie Capers Williams, 29, of Baltimore, and Todd Hiney, 38, whose home town was not given. Vickery said Williams had been convicted of credit card fraud and had tried earlier to escape by impersonating a staff member. Vickery said Hiney had been convicted of robbery.

As of early this morning, both men were still at large.

Staff writer Steven Gray contributed to this report.