Trump Administration Wants Investigation of 'Physical Threat' in NYT Op-Ed

President Trump's administration's come up with a clever way to sniff out the source of that NY Times anonymous letter -- it involves what they perceive as an alleged physical threat to POTUS ... TMZ has learned.

Sources connected to the White House tell us there are multiple West Wing staffers who believe the op-Ed letter -- reportedly written by a "senior administration official" -- contains one troubling sentence that reads like a threat to Trump's life. The line goes ..."So we will do what we can to steer the administration in the right direction until -- one way or another -- it's over."

We're told the staffers think the Secret Service needs to investigate because that line could refer to a possible assassination plot. However, at least one source told us the real endgame is finding out who's behind the letter ... which the Prez has called "gutless."

Truth is that line could refer to any of 3 legit ways for a U.S. President to leave office -- impeachment, the 25th Amendment, or resignation. Then again, we know the Secret Service has investigated for less ... including perceived threats from Kathy Griffin and "Stuttering John."