Joseph, who was helping negotiate nuclear issues with Iran, for more than two years sent hundreds of anonymous and abrasive tweets using the handle @NatSecWonk.
As the widely followed @NatSecWonk, Joseph speculated anonymously about the political motives and career moves of administration officials he worked with. They included Ben Rhodes, President Barack Obama's spokesman on national security issues.
This month, Joseph tweeted that Hillary Clinton, the former secretary of state, "had few policy goals and no wins" in the Middle East. He agreed with Republican Representative Darrell Issa, who has relentlessly pursued Clinton for administration actions after last year's attack on the U.S. diplomatic post in Benghazi, Libya.
"Look, Issa is an ass, but he's on to something here with the @HillaryClinton whitewash of accountability for Benghazi," he tweeted.
Joseph told the website Politico he regretted his tweets.
“It has been a privilege to serve in this Administration and I deeply regret violating the trust and confidence placed in me,”
“What started out as an intended parody account of DC culture developed over time into a series of inappropriate and mean-spirited comments. I bear complete responsibility for this affair and I sincerely apologize to everyone I insulted.”
He also targeted journalists, including Daily Beast reporter Josh Rogin who broke the story of his firing. "Just a hunch, but I have the sense lots of people would like to punch @joshrogin in the face," he said earlier this month.
Officials also told Joseph they suspected he was responsible for a second anonymous Twitter account “@dchobbyist” which included racier tweets about sexual encounters, escort services – and the inner workings of the State Department, according to Politico.