MOGADISHU (Reuters) - A loud explosion followed by gunfire was heard in the Somali capital Mogadishu on Wednesday, Reuters witnesses said, but the cause of the blast was not immediately known. Somalia's Al Qaeda-linked al Shabaab insurgents have in the past claimed responsibility for such attacks. On Sunday, the group's fighters detonated a car bomb in the capital and shot their way into a national intelligence agency training site. (Reporting by Abdi Sheikh; Writing by George Obulutsa; Editing by James Macharia)