I heard this story on the BBC International morning news, which is the only place any Americans are likely to hear about it, despite the fact that it has been authenticated by US officials:

An inmate at the US detention centre in Guantanamo Bay slashed his throat with a sharpened fingernail, US officials have confirmed.[..]

US officials characterised the incident as an act of "self-harm" rather than a suicide attempt.

...US Navy Cmdr Andrew Haynes said there was "an impressive effusion of blood" but the prisoner was treated by guards and taken to the prison clinic.

Officials would give no details of the man but lawyer Zachary Katznelson said the inmate had been held without charge for nearly six years.

Cmdr Haynes said "self-harm" incidents were a tactic to discredit US forces.