Investigators on special counsel Robert Mueller's team have been asking questions about the 2016 Republican National Convention, Reuters reported, citing two sources.

According to one of those sources, the investigators have asked questions about an event connected to the convention that both now-Attorney General Jeff Sessions and then-Russian ambassador to the U.S. Sergei Kislyak attended. But another source, according to Reuters, claims investigators also want to know why the GOP apparently eliminated a section of its party platform that was tough on Russia over its presence in Ukraine.

Mueller, that same source also told Reuters, is interested in whether Sessions had conversations with Kislyak in April 2016 at the Mayflower Hotel in Washington, D.C., when Mr. Trump gave a campaign speech.

Representatives for Mueller's team and Sessions did not respond to requests for comment from Reuters.

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Mr. Trump has repeatedly expressed his frustration over the continuation of the investigation into Russian election meddling and any ties to the Trump campaign, and his now-former lawyer, John Dowd, earlier this month called for the probe to end.

Dowd has since resigned as a lead lawyer for Mr. Trump. But Mueller's investigation doesn't appear to be ending anytime soon. As CBS News' Paula Reid has reported, Mueller's investigation is expected to stretch into 2019. The prosecution of Paul Manafort, Mr. Trump's former campaign chairman, isn't expected to start until September.