Special counsel Robert Mueller may have already subpoenaed President Donald Trump, according to court docket entries that have already been filed, former federal prosecutor Nelson W. Cunningham writes in a column for Politico.

"Since mid-August, he may have been locked in proceedings with Trump and his lawyers over a grand jury subpoena – in secret litigation that could tell us by December whether the president will testify before Mueller’s grand jury," said Cunningham.

He noted that on Aug. 16, a sealed grand jury case was initiated in D.C. federal district court before Chief Judge Beryl Howell, and on Sept. 19, Howell issued a ruling.

The docket sheets do not identify the witness, said Cunningham, and although Politico asked attorneys such as Jay Sekulow who the witness is, they all denied knowledge.

However, he noted that the parties and judges all moved quickly; the appeals court responded with "remarkable speed" and even though there was an "unspecified procedural flaw," the matter was resolved in less than a week, when in other cases such a flaw would have taken weeks or months to resolve.

He added that the evidence points to the witness being the president, including the fact that his sole appointee to the D.C. Circuit court, Gregory Katsas, one of Trump's former deputy White House counsels, has recused himself.

"If Mueller were going to subpoena the president – and there’s every reason why a careful and thorough prosecutor would want the central figure on the record on critical questions regarding his knowledge and intent – this is just the way we would expect him to do so," said Cunningham.