WASHINGTON (Reuters) - Senators Elizabeth Warren and Sherrod Brown on Thursday urged the U.S. Treasury to investigate whether any U.S. or U.S.-linked entity was involved with the law firm at the heart of the “Panama Papers” leak on offshore wealth.  “As the primary agency charged with protecting the integrity of the U.S. financial system and enforcing our laws against money laundering and terrorist financing, we strongly urge the Treasury Department to conduct its own inquiry into Mossack Fonseca’s activities and its clients,” the senators, both Democrats, wrote in a letter to Treasury Secretary Jack Lew. 