CHICAGO (Reuters) - Four Pakistanis have been charged as co-conspirators in the 2008 Mumbai attack that killed 166 people, including six Americans, US prosecutors said on Monday. The four were previously mentioned, but not named, in indictments charging American David Headley and Pakistani-born Chicago businessman Tahawwur Rana with helping to identify targets in Mumbai. Headley and Rana have also been charged in a plot to attack a Danish newspaper that was never carried out. Headley pleaded guilty in March 2010 and is cooperating with US investigators about taking several trips to India - and later to Denmark - to scout targets for the coordinated and lethal assault. Rana has been held since his arrest in 2009 as a conspirator with Headley, and his US trial is scheduled to begin May 16. His attorney was not immediately available for comment. All of the four newly-indicted figures are linked to the Lashkar-e-Taiba. They are: Sajid Mir, Abu Qahafa, Mazhar Iqbal, and a fourth defendant known only by the alias Major Iqbal. None are in US custody. All four are believed to be in Pakistan. They were charged with six counts of aiding and abetting the murder of US citizens and other charges related to the Mumbai attack and providing support.