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WASHINGTON — Special counsel Robert Mueller's team has issued a subpoena to the Trump campaign asking multiple campaign officials to produce Russia-related documents, according to a source with first-hand knowledge of the matter.

The subpoena was first reported by The Wall Street Journal, which also said that the subpoena does not compel testimony by any of the officials.

The source said the campaign has already turned over some 20,000 documents voluntarily, so it’s not clear why the subpoena was issued. A subpoena ratchets up the stakes for noncompliance.

The Journal reported that the Trump campaign had been caught by surprise by the subpoena.

Mueller, a former FBI director, and his investigators are probing any collusion between the Trump campaign and Russia during the 2016 presidential election.

Congressional committees that are also investigating possible collusion had asked the the campaign to turn over Russia-related material dating back to the beginning of Trump's campaign in summer 2015.

On Thursday, the Senate Judiciary Committee said that Jared Kushner, Trump's son-in-law and senior adviser, had failed to turn over all the documents it had requested.