The St. Louis County prosecutor’s office is taking an unusual approach with grand jury members who are weighing evidence against the police officer who shot and killed 18-year-old Michael Brown last month, experts and county officials said.

Instead of telling grand jury members what charges they believe police officer Darren Wilson should face, they are leaving it open-ended for now and involving the grand jury as co-investigators.

The prosecutor’s office is also presenting evidence to the grand jury as soon as it receives it, rather than waiting until the St. Louis County Police Department and the FBI have completed their investigations. Police probes are typically completed before a case is presented to a grand jury, county officials said.

As a result, jurors in the Wilson case are hearing from every eyewitness, seeing every telling photo, viewing every relevant video, and reviewing all DNA, ballistics and other test results from county and FBI labs, said Ed Magee, a spokesman for county prosecutor Robert McCulloch. They will hear testimony from Dorian Johnson, the friend who was with Brown when he died, but it is unclear yet whether they will hear testimony from Wilson.

“Normally they hear from a detective or a main witness or two. That’s it,” Magee said. “This gives us an opportunity to present all of the evidence to jurors who represent St. Louis County. They will make the decision.”

Susan McGraugh, a criminal-defense lawyer and a professor at the Saint Louis University School of Law, said that the approach is allowed under the law and that prosecutors sometimes use it in high-profile cases.

“The prosecutor may want cover, which they can get by sharing the responsibility with the grand jury,” McGraugh said. “So when the public reacts to what does or does not happen, they can go back to the fact that the grand jury played a large role in the decision. They can say, ‘We let these jurors, who are your peers, hear what witnesses had to say. This was their decision.’ “

Pressure from the community for swift action and proof that a thorough investigation is underway also prompted McCulloch to move forward within days of the shooting, rather than waiting weeks or even months for county police and FBI to complete their separate investigations.

Magee said the dual probes are “winding down,” but it is unclear how many additional weeks it will take before they are complete.

The jurors, who were already empaneled at the time of the shooting, meet every Wednesday. Their term is set to expire this week, but they will continue to meet on the Wilson case until they have determined if they will indict the officer.

Wilson could face charges of murder or manslaughter, or the jury could determine that he acted in self-defense or had to use deadly force because he felt his life or the lives of others were in jeopardy. Based on the facts known so far, legal experts say it is very unlikely that Wilson will be charged with first-degree murder, which requires evidence the officer maliciously planned and set out to kill. Second-degree murder charges are possible but also unlikely for a police officer who argues that he feared for his life. If jurors think Wilson used flawed and negligent judgment in deciding to shoot Brown, the officer could face lesser charges of voluntary or involuntary manslaughter, experts suggest.

The grand jury could decide not to indict Wilson at all. The law that determines when police can use deadly force generally gives officers considerable leeway in making that split-second decision about whether they need to kill to save themselves or others. Law enforcement experts say the legal standard, established by two Supreme Court rulings from the 1980s, has made it hard for prosecutors to obtain convictions in cases of alleged use of excessive force.

Under the laws that apply to grand juries, court and other government officials are required to protect the identities of members. In the case of the Wilson grand jury, their identities have remained a secret. County officials have agreed to release information about only their gender and race. The panelists include a black man, two black women, six white men and three white women, court officials said.

The racial makeup reflects that of the county, which is 70 percent white and 23 percent African-American, with all other races being 3 percent or less.

In Ferguson, where last month’s shooting by Wilson, a white officer, of Brown, a black teenager, set off riots and more than a week of racially charged protests, the population is 29.3 percent white and 67.4 percent black, with all other races being 1 percent or less.

“It’s not a secret to anyone in the world that St. Louis has some racial issues,” said Patricia Bynes, Democratic committeewoman of Ferguson Township. “There is a concern that there may be some racial bias” on the grand jury. “With the jury being overwhelmingly a white male jury, it does give us a little pause. It feels like business as usual in St. Louis County.”

Bynes and other black leaders have expressed some skepticism about McCulloch’s willingness to prosecute Wilson. St. Louis County Executive Charlie Dooley has taken the lead in calling for McCulloch, who is white, to recuse himself from the case part because of personal experiences he has had in his life.

McCulloch’s father was a St. Louis police officer when he was gunned down July 2, 1964, at age 37 while trying to arrest a kidnapper. The man who shot him was black.

McCulloch declined to step aside, but he is not presenting evidence to the grand jury, which usually means he won’t try the case himself, Magee said. But he is supervising the two attorneys in his office — Sheila Whirley and Kathi Alizadeh — who are presenting to the grand jury.

Whirley has been with the office since 2001 and is the highest-ranking African-American among the five black attorneys in the 57-member office. Alizadeh has been with the office since 1988 and is supervising the sex crimes unit. She is white.

Alizadeh is also presenting evidence to the grand jury and will prosecute the case if they decide to indict Wilson.

The prosecutor’s office declined to allow either attorney to be interviewed.

Although Alizadeh oversees the sex crimes unit, lawyers said her experience is broad.

“She is a seasoned trial attorney who has done murders, robberies,” said McGraugh, who has been a criminal-defense lawyer in the region for 25 years. “I’ve tried several cases against her, and I was only assigned the most serious felony cases in St. Louis County.”

Rose Ann Feldman, who was an attorney in the county prosecutor’s office with Alizadeh for seven years, said her former colleague is “courageous” and “tough.”

“She is experienced in every kind of crime that is charged,” Feldman said. “She is very competent, very smart.”