James Culp said Wednesday in an email that if the negotiations are successful, Russell would plead guilty to five counts of intentional murder.
The lawyer says he would expect the prosecution to seek a sentence of life without parole.
Russell is accused of going on a shooting spree at the Camp Liberty Combat Stress Center near Baghdad in May 2009.
His defense team has said he has no memory of the killings, and witnesses have said he appeared distant and paranoid before the attack.
He is being held at Joint Base Lewis-McChord