By Robert Patrick

photo: Jamelle Bouie

Ferguson police Officer Darren Wilson testified for almost four hours Tuesday in front of a St. Louis County grand jury investigating the Aug. 9 shooting of Michael Brown, a source with knowledge of the investigation said Wednesday.

Wilson was not obligated to testify, and has also spoken with St. Louis County investigators twice and federal investigators once, the source said. The source said that Wilson had been “cooperative.”

Ed Magee, a spokesman for St. Louis County Prosecuting Attorney Robert P. McCullochs office, said Wednesday that he would not comment on which witnesses have testified.

Tiffany Mitchell, a witness who has spoken publicly about what she saw, has not yet been subpoenaed, her lawyer Peter Cohen said Wednesday. Asked the same question last week about witness Piaget Crenshaw, lawyer Karen Lewis declined to comment.

McCulloch has said that prosecutors would present all the evidence and witnesses to the 12 members of the jury and leave the decision of whether to charge Wilson up to them. Magee told the Post-Dispatch Monday that prosecutors will help navigate legal issues.

Read More Darren Wilson testifies in front of grand jury : News.