McCaskill, a former prosecutor in Missouri, said she's hopeful the physical evidence in the case including blood spatter patterns, clothing and shell casings will provide "incontrovertible facts" about what happened during the shooting. She said whatever local prosecutors decide, it will be important to explain the decision by providing that physical evidence, and that won't be possible if the federal investigation is ongoing.
McCaskill said she urged Attorney General Eric Holder during a meeting earlier this week to speed up what is typically a lengthier federal process.
"What we want to avoid is a decision being made without all the information being available to the public also," McCaskill said, adding that not being able to do so could "create more stress and certainly much more fear that we would be back to worrying about people being able to protest safely."
"Obviously all of us are concerned not just about that this process be fair, but what does this next six months look like?" she said.
Many residents who live in Ferguson, eager to end the disruptions to their lives caused by protests and police presence, say they fear the community's anger will explode anew if Wilson isn't charged.
"This officer has to be indicted. I'd hate to see what happens if he isn't. The rioting, the looting, man ...," said resident Larry Loveless, 29, as he stopped at the memorial for Brown where he was killed.
St. Louis County prosecutors this week convened a grand jury to begin hearing evidence in the case, despite concerns among some in the community including Brown's parents that the office would not be impartial because of District Attorney Bob McCulloch's ties to law enforcement. McCulloch's father, mother and other relatives worked for St. Louis police, and his father was killed while responding to a call involving a black suspect. He has said he will not remove himself from the case.
Considering the racial tensions of the case, even the makeup of the grand jury was being closely scrutinized. Two black women and one black man are on the 12-member panel, along with six white men and three white women, said Paul Fox, director of judicial administration for St. Louis County Circuit Court.
Without specifically mentioning the grand jury's racial makeup, the Brown family's attorney, Benjamin Crump, said the panel "works perfectly" as long as the prosecutor presents the necessary evidence and doesn't withhold information.
Crump said the family doesn't believe the grand jury was necessary, arguing that the prosecutor had sufficient grounds to charge the officer on his own.
The Department of Justice opened a parallel investigation, sending dozens of FBI agents to Ferguson to question witnesses.
Finishing both investigations simultaneously would be unusual because federal investigators typically work independently of their state counterparts and at their own pace, said Paul Cassell, a former federal judge who is a University of Utah law professor.
"That is one of the advantages of a federal investigation. They tend to have a little more distance from the police officers who are being investigated. That provides some assurance of objectivity," he said.
He said prosecutors must avoid considering the potential reaction even a violent one on whether to file charges in any case. He said they must make "a dispassionate judgment uninfluenced by public opinion."
The most likely state charges that will be considered in such a case include second-degree murder, voluntary manslaughter and involuntary manslaughter, he said.