Secret service set to investigate Kathy Griffin's Trump decapitated head photo shoot.

Kathy Griffin may have been joking when she posed with what looked like the bloody head of Donald Trump, but the Secret Service isn't laughing.

Law enforcement sources tell us the Secret Service has launched an investigation into "the circumstances surrounding the photo shoot."

TMZ first posted the pic, shot by famed photog Tyler Shields. We're told the Secret Service does not jump to conclusions that incendiary photos or writings are serious or in jest.

They also will not automatically pass the photo off as art. They routinely investigate and that's exactly what they'll do in this case. We're told agents will most likely attempt to interview her and Tyler.  Kathy apologized Tuesday evening for the images.

TMZ.