USâu00c2u0080u00c2u0088special counsel Robert Mueller is leading the probe into alleged ties between Russia and Trump. Pic/AFP

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 said on Thursday, signalling an investigation is gathering pace into suspected Russian meddling in the 2016 US election.