Special Agent Kelly Bryson said Sgt. Arthur Kaufman -- one of five police defendants accused of covering up the killings -- met voluntarily with federal agents in 2009 to give his version of the incident, but "there were some very specific areas that just didn't make sense to me."
Bryson said she expected the FBI to focus on police use of force, but Kaufman's statements convinced her the police investigation itself should be a target.
Two people were killed and four wounded by police gunfire on the bridge Sept. 4, 2005, as the city reeled from Hurricane Katrina.
Bryson, who headed the civil rights squad in the FBI's New Orleans division, said Kaufman described a supposed eyewitness, Lakeisha Smith, so completely, "you could almost see her in front of you."
Prosecutors say Smith does not exist, and other officers say Kaufman invented her.
Kaufman's lawyer, Stephen London, said Bryson and the FBI were unfairly second-guessing an investigation amid a disaster years after the fact.