Special counsel Robert Mueller’s investigators reportedly issued a subpoena to President Donald Trump’s campaign last month, requesting Russia-related documents from over a dozen top officials, reports the Wall Street Journal.

The subpoena, issued in mid-October, reportedly requested documents and emails with keywords related to Russia but didn’t compel any officials to testify before Mueller’s grand jury, an unnamed source told the Journal.

This is the first report of Mueller compelling the Trump campaign to hand over information.

Jorge Silva / Reuters President Donald Trump and Russia's President Vladimir Putin attend a photo session at a summit in Vietnam on Saturday.

Earlier this year, U.S. intelligence agencies concluded that Russia sought to interfere with the U.S. presidential election to help Trump win.

Since May, Mueller, a former FBI director, has been leading an investigation into possible coordination between the Trump campaign and Russian officials. Trump has repeatedly denied that his team colluded with Russia to win the election.

In late October, a federal grand jury filed the first charges in Mueller’s investigation. Former Trump campaign chairman Paul Manafort and his business associate Rick Gates were indicted ― and have pleaded not guilty ― on charges of conspiracy and money laundering.