Since the attacks on Sept. 11, 2001, anti-terrorism plans have been laid out for post offices and preschools, airports and football stadiums. But what about jails?

Considering that large lock-ups could be targets -- as they have been in other countries -- Fairfax County staged a terrorism-related evacuation drill yesterday, an exercise believed to have been one of only a few held in the United States.

With more than 150 volunteers pretending to be inmates and the real inmates locked down for the day, the drill ran from the moment a deputy opened a faux anthrax letter outside a cell door to the shutting of air ducts and the loading of chained "inmates" onto vans to other jails.

Devising a plan to evacuate 1,200 to 1,300 people -- roughly the daily population of the Fairfax Adult Detention Center -- who may or may not want to cooperate and may or may not try to flee involved a maze of logistics. That included designating assembly areas for evacuated inmates based on how dangerous they would be -- temporary offenders outside the jail and longer-term ones off site.

For inmates sent to the parking garage next door, logistical concerns included supplying portable toilets and arranging for food, nighttime lighting and a way to dispense medicines. For inmates considered more dangerous -- who would be sent in case of an attack directly to another correctional facility, rather than a less secure setup such as the garage -- it meant finding people who know how to drive large trucks and who would be willing to show up to transport inmates who may have just emerged from a contaminated building.

"Some of these things you may not know until it actually happens," said Mike Jackson, who headed the Fairfax jail for 10 years before starting a National Sheriff Association training program last year for terrorism-related jail evacuations.

Jackson and others in attendance yesterday said that jails and prisons may not have the symbolic impact of a target like the Capitol, but they may have their own value to terrorists.

The possibility of inmates escaping could terrorize a community, tie up law enforcement officials and deny the use of jails or prisons. Attacking a correctional facility also could be a way to get at a terrorist informant.

"The question came up recently: What about Abu Ghraib?" Jackson said of the Iraqi prison where U.S. guards have been accused of mistreating detainees. "Would a terrorist strike a jail to get back at us for what we did?"

Jackson, who teaches correctional officials to think about how they would handle terrorist attacks, said the only drill like Fairfax's that he knows of took place in Monmouth County, N.J., last year.

That is not to say that jails and prisons have not been evacuated. Fires and tornadoes have forced many of them to be emptied.

Many states and some accreditation groups also require jails and prisons to have and test fire drills, although such drills are on a much smaller scale, Jackson said.

But since Sept. 11, 2001, more people have begun thinking of jails as possible terrorist targets. Fairfax law enforcement officials said the only time they could remember that their jail had been a target was in 1997, when Mir Aimal Kasi was being held there on charges of killing two people outside CIA headquarters in 1993. The jail received a letter reflecting detailed knowledge of where Kasi was housed and threatening violence if he were not treated well. After that, officials put up large concrete planters to protect the facility, said Lt. Tony Shobe, sheriff's department spokesman.

How would inmates react in case of an evacuation? Lt. Col. David Lubas, head of the jail's operations and yesterday's drill, said he did not think that would be a problem.

"We recognize some people would try and take advantage of the situation, but we feel the average inmate would realize we were there to help save them," Lubas said.