Police have arrested a suspected gunman who opened fire at a security checkpoint outside the White House Correspondents' Dinner attended by US President Donald Trump. Secret Service agents evacuated Trump and cabinet members as thousands of attendees ducked for cover when shooting began. One officer was struck but protected by a bulletproof vest. Authorities say the heavily armed suspect, a hotel guest, was acting alone and is now in custody. This marks the third assassination attempt against Trump, following incidents in Butler, Pennsylvania, and Florida. Multiple investigations are underway into security failures at the high-profile event. Al Jazeera's Alan Fisher reports from Washington DC.