Dickey's addition is particularly notable because he is the first publicly known member of the team specializing solely in cyber issues. The others' expertise is mainly in a variety of white-collar crimes, including fraud, money laundering and public corruption, though Mueller also has appellate specialists and one of the government's foremost experts in criminal law.
Zainab Ahmad and Brandon Van Grack have handled some cybercrime issues in the past, though they are recognized more for their work on terrorism and national security.
Mueller was appointed in May to investigate any possible links or coordination between the Russian government and the Trump campaign to influence the 2016 election, and any matters that might arise out of that work.
He has charged or negotiated plea deals with four former Trump campaign or administration officials, including former national security adviser Michael Flynn and former campaign manager Paul Manafort, though Manafort's charges have nothing to do with his work for Trump.
Some in the legal world had wondered why Mueller had not previously tapped a cyber prosecutor to join his team.
Trump has long denied any wrongdoing and has decried the probe as a "witch hunt." He tweeted Wednesday: "The single greatest Witch Hunt in American history continues. There was no collusion, everybody including the Dems knows there was no collusion, & yet on and on it goes. Russia & the world is laughing at the stupidity they are witnessing. Republicans should finally take control!"
Dickey, who previously worked as an assistant U.S. attorney in the Eastern District of Virginia, has participated in a number of high-profile computer-crime prosecutions — including the ongoing case against the file-sharing site Megaupload and the investigation of the Romanian hacker known as "Guccifer."
Guccifer, whose real name is Marcel Lehel Lazar, penetrated email, Facebook and other online accounts of celebrities and political figures, including former secretary of state Colin Powell, former Bill Clinton aide Sidney Blumenthal and a family member of George W. Bush. His hacking inadvertently revealed that Hillary Clinton was using a private email account when she was secretary of state.