A US Secret Service agent assigned to President Barack Obama’s security detail has died in an apparent suicide, law enforcement sources have said. Special agent Rafael Prieto was being investigated for failing to report a romantic relationship with a foreign national, they told US media. The agency said it was “mourning the loss of a valued colleague”. The behaviour of its agents have been under scrutiny since a prostitution scandal in Colombia earlier this year. A number were dismissed after allegations that Secret Service agents hired prostitutes while they were in Cartagena last April to prepare the way for President Obama’s visit to attend the Summit of Americas. Agents are required to inform the Secret Service of any relationship with a foreign citizen to ensure there is no risk to national security. Rafael Prieto, a married father, admitted to investigators that he had been having a long-term affair with a woman from Mexico, sources told the Associated Press (AP).