Edmonton's maximum security prison has been locked down for more thanthree weeks in an attempt to find a handgun guards believe was thrown over the fence.

Inmates have been in their cells almost around the clock since June 24 in a federal facility in Edmonton known as "The Max." Assistant warden Jim McKenzie said he expects the lockdown to be over by the end of the week.

Maureen Collins, a spokesperson for the John Howard Society, said the animosity created during lockdowns can last well beyond the time the lockdown has ended.

"Four weeks is a very long time," she said.

"There are always after-effects from a lockdown, whether it's a long one or a short one— mistrust on each side, anger, resentment."

Hunting for a gun

The lockdown began after someone driving by noticed a person throwing something over the prison wall. A subsequent police investigation suggested it was a gun.

Guards have been searching for a weaponsince and prisoners have been confined to their cells.

Metal detectors and metal-sniffing dogs are being used in the investigation, said McKenzie.

"Other institutions have provided their own trained weapons dogs, as well as all the inmates are scanned through metal detectors in case they may have a weapon on their person," he said.

As the search continues, prisoners are acting out, he said. Some have started small fires to show officials their frustration, but he says none of the prisoners' reactions has resulted in a serious threat.

So far the search has uncovered a number of homemade knives that could be used to stab someone, but no gun has been found.