The special prosecutor assigned to investigate a possible cover-up by Chicago police in the fatal shooting of Laquan McDonald said in court Wednesday that a special grand jury has been impaneled to hear evidence in the case, reports the Chicago Tribune .

Patricia Brown Holmes, the special prosecutor, said the special grand jury will begin issuing subpoenas over the next 30 to 45 days. She did not disclose if the grand jury has heard any evidence yet, but she said she will file documents under seal on Thursday with Leroy Martin Jr., the presiding judge at the Leighton Criminal Court Building.

During the brief hearing before Martin, Holmes said the special grand jury is made up of 26 people – 16 grand jurors and 10 alternates.

Martin named Holmes to look into whether officers lied to justify the October 2014 shooting of 17-year-old McDonald. Holmes' investigation also could extend to police supervisors, according to lawyers who sought the appointment of a special prosecutor.

Holmes has said the investigation could last anywhere from weeks to months before the grand jury decides whether to indict any officers or not.