TEHRAN (Reuters) - A former FBI agent is being held by Iranian authorities, the Financial Times reported on Friday, but U.S. officials said they were still unable to verify the whereabouts of the missing American. Florida resident and ex-FBI agent Robert Levinson went missing while on a visit to the Gulf island of Kish in Iran early in March. His family has not heard from him since and U.S. officials told Reuters they do not have any valid leads. Diplomats fear the case of Levinson could mark a new twist in apparent tit-for-tat detentions involving the United States, Britain and...