Special counsel Robert Mueller has referred a collection of cases to New York federal prosecutors concerning whether several high-profile American lobbyists and operatives failed to register their work as foreign agents, according to people familiar with the matter.

The transfer of the inquiries marks an escalation of Mueller’s referrals to the US Attorney’s Office for the Southern District of New York in the period since he turned over a case involving President Donald Trump’s former personal attorney, Michael Cohen.

Since the spring, Mueller has referred matters to SDNY involving longtime Democratic lobbyist Tony Podesta and his work for his former firm, the Podesta Group, and former Minnesota Republican Rep. Vin Weber and his work for Mercury Public Affairs, the sources said.