Nicholas Slatten was sentenced to life after being found guilty of murder 

Prosecutors say Slatten was the first to fire shots in the September 2007 massacre of Iraqi civilians at a crowded traffic circle in Baghdad

Slatten was convicted of killing Ahmed Haithem Ahmed Al Rubia'y, 19, an aspiring doctor 

In all, 10 men, two women and two boys, ages 9 and 11, were killed

It was Slatten's third trial on the charges. His first conviction was thrown out and the jury was unable to reach a unanimous verdict at his second trial