Mr. Malvo gave Detective Walker a brief tutorial on his Bushmaster rifle.

It is not yet known where or how Mr. Muhammad bought the Bushmaster rifle that the authorities say was used in the killings.

The AR-15 is the civilian version of the military's M-16, and the Bushmaster rifles are copies of it.

That date would have been before Mr. Muhammad could have bought the Bushmaster rifle used in the sniper killings.

The police say they found a high-powered Bushmaster rifle in the car that was traced through ballistic tests to many of the shootings.

Spengler had armed himself with three guns: a .38-caliber revolver, a 12-gauge shotgun, and a .223-caliber Bushmaster semiautomatic rifle outfitted with a flash suppressor.

At one moment, Mr. Malvo did break his pose as a sheriff's deputy used scissors to cut open a box containing the Bushmaster rifle.

Mr. Muhammad, the authorities have said, claimed in a brief interrogation on the day of the arrests that he had found the Bushmaster rifle only a couple of days earlier.

Most important of these was the Bushmaster semiautomatic rifle that investigators say they found hidden behind the rear seat.

He also claimed that the 20 victims who had been killed with a Bushmaster semi-automatic rifle had "defensive wounds."