A grand jury has issued subpoenas in connection with a June 2016 meeting that included President Donald Trump’s son, his son-in-law and a Russian lawyer, two sources told Reuters on Thursday, signaling an investigation is gathering pace into suspected Russian meddling in the 2016 U.S. election.

The sources added that U.S. Special Counsel Robert Mueller had convened the grand jury investigation in Washington to help examine allegations of Russian interference in the vote. One of the sources said it was assembled in recent weeks.

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“When someone gets a subpoena to testify, that can drive home the seriousness of the investigation,” said David Sklansky, a professor at Stanford Law School and a former federal prosecutor.