Video After The Jump
The FBI has questioned a man who says he recorded audio of gunfire at the time Brown was shot by Ferguson, Missouri, police on August 9, the man's attorney told CNN.
In the recording, a quick series of shots can be heard, followed by a pause and then another quick succession of shots.
Forensic audio expert Paul Ginsberg analyzed the recording and said he detected at least 10 gunshots -- a cluster of six, followed by four.
"I was very concerned about that pause ... because it's not just the number of gunshots, it's how they're fired," the man's attorney, Lopa Blumenthal, told CNN's Don Lemon. "And that has a huge relevance on how this case might finally end up."
The man, who asked that his identity not be revealed, lives near the site of the shooting and was close enough to have heard the gunshots, his attorney said.
He was speaking to a friend on a video chat service and happened to be recording the conversation at the same time Brown was shot, Blumenthal said.
The attorney said she learned of the man's recording late last week from a mutual friend.
"I had to get his consent before I could reach out to the FBI," Blumenthal said.
CNN cannot independently verify the authenticity of the tape and has asked the FBI for confirmation of their interview with the man who made the recording.
The meaning of the pause
It's difficult to prove from the audio why the pause took place or whose narrative it supports.
Attorney Chris Chestnut said he was surprised by the gap in shots.
"It's the pause that gives most concern in a police shooting, especially with an unarmed victim, because at this point Mr. Brown is defenseless -- he has no weapon," said Chestnut, who represented the family of Jonathan Ferrell.
Like Brown, Ferrell was an unarmed African-American man who was shot and killed by a white police officer.
But if the gunfire heard on the audio is indeed from the Brown incident, the pause doesn't automatically suggest wrongful intent by the officer.
"To be fair, there could be other explanations for that pause," said attorney Van Jones, co-host of CNN's "Crossfire." "Maybe the officer will say, 'Well, I fired and he kept advancing, so I fired again.' "
Witnesses and a friend of Officer Darren Wilson have given conflicting accounts of what led up to Brown's death.
Dorian Johnson, a friend of Brown's who was walking with him at the time of the shooting, said the officer shot Brown once by the police car and again as he ran away.
According to Johnson, Brown was struck in the back and then turned around and put his arms up as the officer kept shooting.
But a friend of Wilson said Brown mocked the officer and charged at him before the shooting began.
An autopsy showed that all the entry wounds were in the front of Brown's body.
Key witness speaks out
Ferguson police said Brown allegedly robbed a convenience store shortly before the shooting.
And reports that his friend Johnson had a criminal record that includes lying to police has put Johnson's credibility in question.
In 2011, Johnson was arrested and accused of theft and lying to police about his first name, age and address.
Johnson said Monday night he doesn't understand why some are questioning his credibility.
"I see they bring up my past, my history, but it's not like it's a long rap sheet," Johnson told Lemon. "This one incident shouldn't make me a bad person."
CNN's Eliott C. McLaughlin, Tina Burnside, Julian Cummings and Greg Botelho contributed to this report.