NEW YORK (AFP) US federal prosecutors have launched a criminal corruption probe into one of Afghan President Hamid Karzais brother, The Wall Street Journal reported Monday. Mahmood Karzai, a US citizen subject to US tax law, is at the centre of an investigation led by federal prosecutors in the Southern District of New York, the newspaper said, citing US officials. The move could risk a further setback in already toxic ties with the mercurial leader, who has challenged US efforts to overcome graft in Afghanistan that they say is undermining public support for the Afghan president and thus helping fuel the Taliban insurgency.