Anderson Cooper's Assistant Partially Responsible for Trump-Bashing Tweet

CNN says it's gotten to the bottom of Anderson Cooper's Trump-bashing tweet ... and all signs point to a forgetful assistant.

The mystery was cleared up Wednesday when the network said the message was sent from Cooper's assistant in NYC, while Anderson was in D.C.

CNN says the assistant, who's been on the job for 10 years, left his phone unlocked and unattended at the gym, and someone grabbed it and fired off the tweet. He's apparently the only other person besides Anderson with access to the account.

As we reported ... Anderson's verified Twitter account replied to Trump early Wednesday morning -- calling 45 a "tool!" and a "Pathetic loser."