Wyoming prosecutor Cal Rerucha has announced he is seeking the death penalty for Aaron McKinney and Russell Henderson, on trial for the murder of Matthew Shepard, 21.

Henderson and McKinney: Charged with first-degree murder

The two are charged with first-degree murder, kidnapping and aggravated robbery.

For the death penalty to be imposed under Wyoming law, the prosecution will have to prove aggravating circumstances in the killing.

The brutality of the killing in October shocked the US.

Matthew Shepard was tied to a fence, beaten and pistol-whipped, after being lured from a campus bar by two men who told him they were gay.

Mr Shepard was left for dead in freezing temperatures and died in hospital later. His skull was so badly fractured by the beating that doctors said surgery was not an option.

Matthew Shepard: Targetted because he was gay

Police said that although the main motive may have been robbery, the attackers also singled out their victim simply because he was gay.

The savage attack has prompted calls for tougher laws to combat violent crimes based on prejudice, which monitoring groups say are on the increase.

Members of Congress have joined prominent gay activists in expressing outrage and pleading for tolerance.

President Bill Clinton, who sent representatives to Mr Shepard's funeral, urged members of Congress to pass legislation making it easier for the federal prosecution of prejudice crimes.

Several US states have laws targetting crimes of prejudice on their statute books. Wyoming is not among them, though state legislators have called for it to pass an anti-prejudice crime law.