Washington, April 14 (IANS): Four former US Blackwater security guards were sentenced by a federal judge to long prison terms for the 2007 massacre of 14 unarmed Iraqi civilians in Baghdad.

US District Court Judge Royce Lamberth sentenced Nicholas Slatten to life in prison after his murder conviction in the killings at a Baghdad traffic circle, Efe news agency reported on Monday.

Three other former Blackwater guards who had been convicted of manslaughter in the killings -- Paul A. Slough, Evan S. Liberty and Dustin L. Heard -- were each sentenced to 30 years behind bars.

Slatten is a former US Army sniper.

The four former Blackwater employees were found guilty by a federal court last October, although their attorneys appealed their convictions without success.

According to court documents, the men, who were tasked with providing security for US government employees in Iraq, opened fire on pedestrians with machine guns and grenade launchers at Nisur Square on September 16, 2007, during an operation with the intention of clearing a road for the passage of a State Department convoy shortly after a car bomb exploded in the vicinity.

Fourteen Iraqis were killed in the shooting and 18 were wounded.

The case forced a revision of the US government's relationship with the North Carolina-based Blackwater, which -- in the face of the criticism it received for the excessive use of force and its links with other cases of illegal weapons export -- changed its name twice, first to Xe and later to Academi.

Between 2002 and 2012, Blackwater received millions of dollars in private US government contracts to protect convoys, buildings, military installations and officials of countries where the US armed forces were at war.