UPDATE:
CNN reports Monday morning:
Kellyanne Conway says she was with Trump "all day" Saturday and that John Dowd crafted the Flynn tweet and sent it to the "director of social media."
— Kaitlan Collins (@kaitlancollins) December 4, 2017
At the same time, on June 6, 2017, then-press secretary Sean Spicer said this of the president's tweets:
"The president is president of the United States," Spicer said, "so they are considered official statements by the president of the United States."
Trump’s Tweets ‘Official Statements,’ Spicer Says (NBC News)
John Dowd, President Trump's personal attorney, says he wrote the tweet in which President Trump seems to admit to obstruction of justice:
I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!
— Donald J. Trump (@realDonaldTrump) December 2, 2017
CNN:
Dowd said he drafted the tweet and believes White House social media director Dan Scavino posted it online. He declined to answer additional questions about whether Trump reviewed the tweet before it was posted.
"Enough already," he said in an email. "I don't feed the haters."
The tweet is problematic because if Trump knew Flynn lied to the FBI, asked FBI Director James Comey to let Flynn go, and then fired Comey, he would be guilty of obstructing justice.
The Washington Post first reported Dowd's authorship, citing two unidentified sources. CBS News followed suit:
John Dowd was the principal wordsmith for the tweet, and it's unclear whether Mr. Trump saw the final version, sources said. The president was in between fundraisers in New York when the tweet was sent from an iPhone.
(...)
Dowd meant to convey that Flynn was fired for lying to Pence and other White House officials about his conversations with Kislyak, and "now we know" that he lied to the FBI, according to these sources. But Dowd bungled the tweet and created confusion around Mr. Trump's knowledge of events.
Dowd also told ABC News he wrote the tweet and in a "sloppy" way, but ABC notes the administration is not commenting and someone accessing the president's account and tweeting without his approval would be odd:
The White House, which has insisted that tweets from the president should be taken as official statements, has declined to comment.
It's unclear who actually tweeted out the message; it is unlikely Dowd has direct access to the president's official Twitter account and could have tweeted without his knowledge or involvement, sources said.
Trump's lawyer says he was behind President's tweet about firing Flynn (CNN)
Trump on Michael Flynn’s guilty plea: It’s a ‘shame’ because he had ‘nothing to hide’ (WaPo)
Trump lawyer wrote tweet implying Trump knew Flynn lied to FBI (CBS News)
Trump's lawyer: I wrote the president's 'sloppy' tweet about Flynn's dismissal (ABC News)