WASHINGTON - A federal judge sentenced four former Blackwater security guards to lengthy prison terms on Monday in the 2007 massacre of 14 unarmed Iraqis, closing a chapter of the US war in Iraq that tested relations between the two countries.

Nicholas Slatten was sentenced to life in prison for his murder conviction in the killings at a Baghdad traffic circle.

US District Court Judge Royce Lamberth sentenced three other former Blackwater guards, convicted of manslaughter in the killings, to 30 years each.

The four guards opened fire with machine guns and grenade launchers on the unarmed Iraqi civilians, including women and children, at Nisur Square on Sept 16, 2007. A heavily armed, four-truck Blackwater Worldwide convoy had been trying to clear a path for US diplomats.

In addition to the killings, at least 17 Iraqis were injured.

Slatten was convicted in October, along with ex-guards Paul Slough, Evan Liberty and Dustin Heard.

North Carolina-based Blackwater was sold and renamed several times. It is now called Academi, based in northern Virginia.