The FBI will open a parallel investigation into the fatal shooting of Michael Brown, 18, by Ferguson, Mo., police on Saturday.

St. Louis County Executive Charlie Dooley made the announcement in a late-morning press conference Monday.

He promised the process would be open but not quick.

"We're asking all the public to be calm, be patient," Dooley said.

St. Louis County Police Chief Jon Belmar says the FBI investigation will be a parallel investigation.

"The bureau will take a lot of the information we have developed already through our investigation," Belmar said. "They will take that investigation and they may do their own interviews and take a look at the evidence themselves."

"We will share our information with Prosecutor McCulloch's office and at the same time with the FBI," Belmar said.

Belmar said Ferguson's police chief, Tom Jackson, contacted him Saturday even before he got to the scene of the shooting. "This is a complex investigation, as it should be," Belmar added. "We need to make sure everything's done right. We get one chance to do it."

He said investigators still need to talk to scores of people at the apartment complex.

"I understand the public has a right to be skeptical," Belmar said. "But I would also ask the public to be reasonable."

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