Donald Trump has told James Comey to "remember sailor".
The bizarre message – which appears to be something of a threat – came at the end of a long attack on the ex-FBI director.
"Is everybody believing what is going on. James Comey can’t define what a leak is," Mr Trump posted, in reference to an ongoing argument about Mr Comey's disclosures. "He illegally leaked CLASSIFIED INFORMATION but doesn’t understand what he did or how serious it is. He lied all over the place to cover it up. He’s either very sick or very dumb. Remember sailor!"
The two words at the end prompted widespread confusion and mockery, as people scrambled to understand what he meant. Some suggested it was a strange warping of the expression "hello sailor", or that it was a reference to Japanese series Sailor Moon, which is what came up when searching the phrase on Google.
In fact, the phrase appears to be a veiled threat to prosecute Mr Comey over what the president considers the leaking of classified information.
The phrase seems to be a reference to Kristian Saucier, a Navy submariner who was pardoned by Donald Trump last month. He had been convicted of illegally taking pictures from inside a navy vessel in 2009.
The Saucier case became a key cause for some in the Republican establishment, who said that he had been treated far more harshly than Hillary Clinton. Comparisons between the two were rife, and came up during the election campaign at the vice-presidential debate between Mike Pence and Tim Kaine.
The president appears now to be using that example not only to attack Hillary Clinton but also James Comey.
Mr Trump sent a tweet using the same words in January this year, explicitly referencing Saucier. That appears to confirm that the strange message is a reference to the Saucier case.
"Crooked Hillary Clinton’s top aid, Huma Abedin, has been accused of disregarding basic security protocols. She put Classified Passwords into the hands of foreign agents," Trump tweeted in January. "Remember sailors pictures on submarine? Jail!"
Mr Trump appears to use the sailor case as an example of when distributing classified information can be a crime. It may be that he is suggesting that Mr Comey could be pursued under the same principle – though it is not clear why the president would bring that case back up when Saucier was pardoned.
It is not the first time that Mr Trump has issued a cryptic threat to James Comey through his Twitter.
"James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" he posted in May 2017, after the FBI director had been fired. That was taken by many to be a suggestion that there could be such tapes, and that they might be made public depending on what Mr Comey said to the media.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies