Juba?

They have never seen Juba. They hear him, but by then it's too late: a shot rings out and another US soldier slumps dead or wounded.
There is never a follow-up shot, never a chance for US forces to identify the origin, to make the hunter the hunted. He fires once and vanishes.

This is a quote from an Iraqi pro-insurgent website.

'Juba' is the nickname given by American forces to an insurgent sniper operating in southern Baghdad.
Since February, the killing of at least two members of the 1-64 battalion based in Camp Rustamiyah and the wounding of six more have been attributed to Juba.

Some worry that Juba is on his way to becoming a resistance hero, acclaimed by those Iraqis who distinguish between "good" insurgents, who target only Americans, and "bad" insurgents who harm civilians.

He has supposedly killed at least 12 soldiers.