St. Louis County investigators will not release details about the shooting of Michael Brown until the information is presented to a grand jury, a process that could take months, the U.S. Attorney on the case said Tuesday.

FBI agents are sharing information they have gathered from the area of the shooting scene with St. Louis County investigators, U.S. Attorney Richard A. Callahan in Missouri told TIME, noting "physical evidence is being analyzed" from the autopsy done Monday.

"While the lack of details surrounding the shooting may frustrate the media and breed suspicion among those already distrustful of the system, those closely guarded details give law enforcement the best yardsticks for measuring whether witnesses are truthful,” Callahan said. 

“Without those yardsticks, an investigation becomes more of a guessing game or popularity contest than a search for the truth," he added.

On Monday, U.S. Attorney General Eric HolderEric Himpton HolderPavlich: The claim Trump let the mentally ill get guns is a lie Pennsylvania Supreme Court releases new congressional map 36 people who could challenge Trump in 2020 MORE called for patience during the federal investigation.

“No matter how others pursue their own separate inquiries, the Justice Department is resolved to preserve the integrity of its investigation," said Holder, who plans to visit Ferguson on Wednesday.

The St. Louis County Police Department is also investigating the shooting.

Protests and looting have caused a ramped up police presence in Ferguson since Brown was shot Aug. 9 by police officer Darren Wilson. Many of the protesters are demanding that law enforcement release more information about the shooting.

"That's not going to happen tomorrow. It may be weeks, it may be months, We have to bring calm to our community," Missouri Highway Patrol Capt. Ron JohnsonRonald (Ron) Harold JohnsonTrump spars with GOP lawmakers on steel tariffs Overnight Regulation: Trump unveils budget | Sharp cuts proposed for EPA, HHS | Trump aims to speed environmental reviews | Officials propose repealing most of methane leak rule Trump budget seeks savings through ObamaCare repeal MORE told MSNBC's "The Daily Rundown" on Tuesday.

"Our kids cannot stay in their bed for months and not go to school. Our officers can't come out here for months and put their lives in danger. Peaceful protestors can't come out here for months and put their lives in danger. These homeowners cannot be uncomfortable sleeping in their homes wondering if a stray bullet is going to come into their home," he said. 

"We have to let our legal system work itself out, and that's what makes our country great."